If sinx=.42, calculate the value of cos x?

The sinus and cosinus have fixed values depending on the degrees they're applied for, so first we have to figure out how much is the x:

sin x = 0'42
x = arc sin 0'42
x = 24,83º

Now, if we wanna get the cos x, we just have to calculate cos 24'83, which would be 0,91.