how do u do interpolation and extrapolation?

Answer 1

These are two ways to make estimates of values when you already have some data. To interpolate is to estimate values that is missing BETWEEN two sets of data. This is comonly done graphically with both straight line representations of the data or curved lines.
Extrapolation is to estimate BEYOND what has been described. With straight line , it is simply to extended sraight.
With a curved line the trend of th curve is estimated and the line is extended in estimation of what the curve was doing at its termination.

Answer 2

The simplest way is to do this linearly (though there are others that can be more accurate). To interpolate to a value, draw a line from the value directly below to the value directly above the value you are interpolating to. The point this line crosses your desired value is the interpolated value.
To extrapolate draw a line using the two nearest values of data to the point you want to extrapolate to. This is the value you want. That is the basic principle. To do it matematically, solve for the equation of a line using the two points you need and then plug in the value you want to interpolate/extrapolate to.