Question

The measure of an angle is 51°.

Answer 1

Supplement of an angle is what must be added to that angle to get 180

51+supplement=180
minus 51 both sides
supplement=129

answer is C

Answer 2

Here you need to know that supplement is Equal to 180

Here how u would do it
51+ x=180
solve for x
x=180 - 51
x= 129

So c is the answer to your question