c^2 = a^2 + b^2 - 2*ab*Cos(C)
c = 16; a = 17; b = 8 (what you call a and b don't really matter. c does). Substitute.
16^2 = 17^2 + 8^2 - 2*17*8*Cos(C) Add the first 2 on the right.
256 = 289 + 64 - 282*cos(C)
256 = 353 - 282*cos(C) Whatever you do, don't do any more combing on the right side. Subtract 353 from both sides.
-97 = -282 * cos(C ) Divide by 282
0.34397 = cos(C)
cos-1(0.34397) = C ; C = 69.88 degrees.
Do you need more help on this question? All of these are done the same way.