Hello from MrBillDoesMath!
Answer:
6 sqrt(2)
Discussion:
Call the missing leg "x". Then from Pythagoras
x^2 + 7^2 = 11^2 => subtract 7^2 ( = 49) from both sides
x^2 =121 - 49
x^2 = 72 => take square roots of both sides
x = sqrt(72) => 72 = 36 * 2 = 6^2 * 2
x = 6 sqrt(2)
Thank you,
MrB