The angles in a triangle add to 180º.
This is no different when the angles are algebraic expressions.
Form an equation from the angles shown in this triangle and solve it to find the value of a.
The sum of the angles = 180
a + 10 + a - 37 + a = 180
We simplify and solve this equation as follows:
3a - 27 = 180
3a = 207
a = 207 ÷ 3 = 69
a = 69