Find m∠C if ∠C and ∠D are supplementary, m∠C= 3y-5, m∠D= 8y + 20

Question

This is from a geometry question but i really dont understand it could someone explain it to me i got 40 somehow.

Answer 1

If <C and <D are supplementary m<C + m<D = 180

m<C = 3y - 5

m<D = 8y + 20

substituting for m<C and m<D in top equation:

3y - 5 + 8y + 20 = 180

11y + 15 = 180

11y = 165

y = 15

m<C = 3y - 5 = 3*15 - 5 = 40

m<D = 8y + 20 = 8+15 + 20 = 140