90 degree angle

inside of it is- 7a+12 and 12+a

Which equation could be used to solve problems involving the relationships between the angles?
Responses
A (12 + a) + (7a + 12) = 90(12 + a) + (7a + 12) = 90
B 90 + (12 + a) = (7a + 12)90 + (12 + a) = (7a + 12)
C (12 + a) − (7a + 12) = 90(12 + a) − (7a + 12) = 90
D (7a + 12) − 90 = (12 + a)(7a + 12) − 90 = (12 + a)
E (7a + 12) + 90 = (12 + a)

The correct equation to solve problems involving the relationships between the angles is option D: (7a + 12) − 90 = (12 + a).