Question 433343
By the Pythagorean theorem, {{{r = sqrt(a^2 + b^2)}}}. If you are trying to enter it on an online class/test, you might want to try


sqrt(a^2 + b^2)
Sqrt[a^2 + b^2] if you are using Wolfram Mathematica (or similar) software
$\sqrt{a^2 + b^2}$ if you are using LaTeX


If it doesn't accept any of these formats, you might want to look into your online software's manual on how to input such expressions.