Start by writing what the problem tells you, and what you know.
What you know - the sum of supplementary angles is 180, the sum of complementary angles is 90, and the sum of linear pairs is 180.
What the problem tells you: m>1 + m>2 = 180, m>1 + m>3 = 90 and m>2 +m>3 = 180
There are three equations and three unknowns, so we can solve for all three.
Add the first two equations: m>1 + m>2 + m>1 + m>3 = 180+90 = 270.
We know, by the third equation, that m>2 + m>3 = 180, so substitute that into our added equations:
m>1 + 180 + m>1 = 270
Subtract 180 from both sides: 2*(m>1) = 90 ...divide by 2 m>1 = 45
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