Let s represent the measure of the second angle. Then the sum of the three angles of the triangle is ...
... first + second + third = 180°
... 2s + s + (s+72°) = 180° . . . . substitute the given information
... 4s = 108° . . . . . . . . . . . collect terms, subtract 72°
... s = 27° . . . . . . . . . . . . . divide by 4
The first angle measures 54°.
The second angle measures 27°
The third angle measures 99°.
