To solve this answer, first you have to understand what a supplementary angle is.
What are supplementary angles?
Supplementary angles are angles whose measures add up to 180°
In this case, that means the measures of <A and <B should add up to equal 180°.
To find m<A, we have to first find what x equals.
Add the two equations for <A and <B together and set the final equation equal to 180:
(2x - 9) + (2x +21) = 180
Now simplify the equation by combining like terms:
4x + 12 =180
Isolate the x on one side of the equation by subtracting both sides by 12. This will cancel out the 12 on the left side of the equation and leave the right side of the equation with 192 (180 +12).
Now your equation is 4x = 192
Divide both sides by 4 and x = 48
Now plug 48 into the equation for m<A (2x-9)
m<A = 2(48) - 9
Now solve:
m<A = 87