A straight line is 180°. So:
∠QRT + ∠TRS = 180° [outside ∠R + inside ∠R = 180]
(45x) + ∠TRS = 180 Subtract 45x on both sides
∠TRS = 180 - 45x
A triangle is also 180°, so the three angles add up to 180°.
∠R + ∠T + ∠S = 180° [∠R is also ∠TRS]
(180 - 45x) + (25x) + (57 + x) = 180
180 - 45x + 25x + 57 + x = 180 Combine like terms
237 - 19x = 180 Subtract 237 on both sides
-19x = -57 Divide -19 on both sides
x = 3
