The exterior angle of the triangle equals the sum of the two opposite interior angles of the same triangle.
x= unknown value
9x= 5x + (9 + x)
combine like terms
9x= 6x + 9
subtract 6x from both sides
3x= 9
divide both sides by 3
x= 3
CHECK
9x= 5x + (9 + x)
9(3)= 5(3) + (9 + 3)
27= 15 + 12
27= 27
ANSWER: x= 3
Hope this helps! :)