start off by knowing that the two triangle are congruent. By knowing this you can turn the given numbers into fractions.
9/72 = 7/7x+7
then cross multiply, which equals
63x + 63 = 504
then solve this equation
63x = 441 - subtract 63 from both sides
x = 7 - divide by 63 from both sides
so your answer is x=7
