You use the equation A squared plus B squared equals C squared. Then, you substitute the value that you have, so the equation would be a squared plus 3 squared equals 5 squared. Next, you square the two values to make the equation a squared plus 16 equals 25. After that, you subtract one of the values (which in case is 16) on both that leg value and the hypotenuse, so you are left with A squared equals 9. Finally, you square both 9 and A squared and you are left with A = 3. I hope that helps you understand it clearly!
