Answer
The third side would be $29$.
Work Step by Step
We can find the third side by using the Pythagorean theorem, which states that $a^2 + b^2 = c^2$, where $a$ and $b$ are the legs of the right triangle and $c$ is the hypotenuse.
Let's plug in what we know into the Pythagorean theorem:
$20^2 + 21^2 = c^2$
Evaluate the exponents:
$400 + 441 = c^2$
Add to simplify:
$841 = c^2$
Take the positive square root to solve for $c$:
$c = 29$
The third side would be $29$.
