HELP ME PLEASE! I Don't UNDERSTAND!!

Answer: 4.12
"you should rlly get to know this"
btw ^2 means to the power to 2
Step-by-step explanation:
a^2 + b^2 = c^2
in this case, u don't have "a"
the slanted side is "c"
a^2 + 8^2 = 9^2
a^2 + 64 = 81
a^2 = 81 - 64
a^2 = 17
___
a = _/ 17
a = 4.12