The Pythagorean theorem says the length of the hypotenuse squared is equal to the sum of both of the lengths of the legs squared. To put that visually, look at my attached image.
- hypotenuse definition: longest side of a right-angled triangle or the side opposite the right angle
- legs definition: side other than the one opposite the right angle
Based on the Pythagorean Theorem, we can put the known information, both legs' length is 7, into the equation.
Thus the unknown side length is about 9.899.
Hope that helped!