Can someone describe to me clearly, how you would solve these questions:

1. Angles x and y are located in the first quadrant such that sinx=4/5 and cosy=7/25.

a) determine an exact value for cos x

b) determine an exact value for sin y

c) determine an exact value for sin(x+y)

2. angle x lies in the third quadrant, and tanx=7/24

a) determine an exact value for cos2x

b) determine an exact value for sin2x

for 1 a and b I got it, but for 1 c I am not sure how to approach it