okay so on this line you can see its a complementary angle, meaning the will add up to be 90, as reciprocated, shown on the other side as a right angle.
so how you will want to set it up is as such:
(2x+9)+(x-9)=90
combine and subtract your whole number
3x=90
divide 3 from both sides to get x by itself giving you your answer:
x=30
