Set it up in an equation.
1) 45+5x=9x+9
2) Get x on ONE side: subtract 5x from both sides
3) bring down the equation: 45=4x+9
4) get x by itself: subtract 9 from both sides
5) bring down the equation: 36=4x
6) get x by itself: divide 4 from both sides
7) bring down the equation : 9=x
8) put 9 in the place of x : 9(9)+9
9) 9•9=81
10) 81+9=90
So the exterior angle is equal to 90.