b. How you determine the temperature, composition, and motion of an object from its light spectrum?

Answer 1

To determine (rough) temperature, we use the black-body spectrum. Basically, an object will emit a specific amount of heat (and eventually light) depending on its temperature. Stars are pretty close to this, but other objects will vary.

Composition is 'where' this is done. Certain elements will absorb and emit light at specific frequencies. Determining this is done through a specific means called spectroscopy, which is basically a fancy version of a prism.

You can determine relative radial (but not lateral) motion by the redshifting or blueshifting of the resulted spectrographic data. Since you know what lines hydrogen emits, you can tell what they're supposed to be and, from that, how fast it is moving away from or towards you.

Answer 2

Xeriar is essentially correct. But in the interest of clarity let me expand on his answer.
It is true that any hot solid or hot gas under pressure emits light on all frequencies of the spectrum. Wein's law, however, tells us that the frequency that has the highest level of intensity can tell us what the temperature is. For example, if you were to measure the spectrum of a yellow star, you would note that the light intensity at a certian wavelength is much higher than the other wavelengths, and that would correspond to the temperature of the star.
The composition is also in the spectrum. Every element in the universe has a unique signature in its spectrum. The most famous are the Balmer lines of hydrogen. In other stars, we see lines of helium, and heavier elements. As light passes through or reflects off cooler objects, some wavelengths of light will be absorbed based on the elements present. These emission lines (where lines are radiated) and absorption lines (where lines are subtracted from a spectrum) tell us what elements are present and therefore what the composition is.
The motion of an object in space has two components. Radial and Tangential velocity. A spectrum can tell you how fast an object is moving towards or away from you by the doppler shift (red shifting or blue shifting) of the lines in a spectrum. With a Star, the Balmer lines are supposed to occur at very specific wavelengths. If they do not, then you look to see where they are located. How far they are shifted from the correct position tells you how fast the star is moving. If they are shifted to the red then the star is moving away. If to the blue, then the star is moving closer.
The tangential velocity can not be determined by the spectrum. The tangential velocity is measured over a long period of time watching by observing the motion of the object moving in the sky across the celestial coordinates (Right Ascension and Declination)
Hope this helped.