Answer:
Step-by-step explanation:
Sum of angles inside triangle is 180
b + (a -3) + 90 =180
b + a - 3 + 90 = 180
b + a + 87 = 180
b + a + 87 -87 = 180-87
b + a = 180-87
b + a = 93
b = 93 -a
