Hopefully, you're talking about Pythagorean theorem.
a^2 + b^2 = c^2
They gave us side a and side c, so we plug in our values
10^2 + b^2 = 16^2
You solve the exponents
100 + b^2 = 256
Subtract 100 on both sides
100 + b^2 = 256
-100 -100
You're left with:
b^2 = 156You square root both sides
b = 12.489...
We round and get our final answer:
b = 12.49
Good luck!