Can someone explain how to do this I'm lost.

Step-by-step explanation:
[tex]\huge\bold\purple{ 5 }[/tex]
[tex]\huge\bold\purple{ explanation }[/tex]
[tex]90(right \: angle) = (9x + 1) + (7x + 9) \\ 90 = 16x + 10 \\ 16x = 90 - 10 = 80 \\ x = 80 \div 16 \\ x = 5| [/tex]