You have the following expressions for the two given angles:
(b + 6)°
(3b)°
In order to solve for b, you take into account that the two angles are complementary, that is, they add up 90°. Then, you sum the previous expressions and equal it to 90, just as follow:
(b + 6) + (3b) = 90
In order to solve for b the previous equation, you proceed as follow:
(b + 6) + (3b) = 90 eliminate parenthesis
b + 6 + 3b = 90 subtract 6 both sides
b + 3b = 90 - 6 simplify similar terms
4b = 84 divide by 4 both sides
b = 84/4
b = 21
Hence, the value of b is b = 21