You are still in the cosine law. All that changes is that 17^2 is put on the left by itself.
a^2 = b^2 + c^2 - 2bc * cos(A) Solve for A
17^2 = 8^2 + 16^2 - 2*8*16* cos(A)
289 = 64 + 256 - 256*cos(A)
289 = 320 - 256*Cos(A)
289 - 320 = - 256*Cos(A)
-31 = - 256 * Cos(A)
-31 / - 256 = cos(A)
0.1211 = cos(A)
cos-1(0.1211) = A
83.05 = A
