Suppose you know that ∠s and ∠y are complementary, and that m∠s = 2(m∠y) − 45°. find m∠y.

If S and Y are complementary, that means they add to 90.

So S + Y = 90
And S = 2Y - 45

Use substitution to plug 2Y - 45 in for S.

(2Y - 45) + Y = 90

3Y - 45 = 90
3Y = 135
Y = 45

If Y equals 45, then S also equals 45 since S+Y=90.