Missing Vector (Triangles and cosines in physics)?

Question

(a) Vector a =?
(b) Vector b= 25 N
(R) Resulting Vector= 30 N
Theta= 104degrees

i need to solve for vector 'a' and i put it into the equation

b² + R² - 2bR cos(theta)
or
25²N + 30²N - 2(25)(30) cos(104)

but when i put the answer back into the regular equation:
a² + b² - 2ab cos(theta)

it doesnt work out >.< please help! i do best answers!

Answer 1

Perhaps you should write the Parallelogram Law of Vectors as

R^2=a^2+b^2 + 2 a b cos(104) and that would help.

Put the numbers in and solve for a.