The two angles shown are supplementary angles, meaning that there sum equals 180. Knowing this you can set up an equation and solve for h...
3h + 18 + 15h = 180
Combine like terms
18h + 18 = 180
Subtract 18 to both sides
18h + (18 -18) = 180 - 18
18h = 162
Isolate h by dividing 18 to both sides
18h/18 = 162 /18
h = 9
Hope this helped!
