Using the formula for the Pythagorean theorem a^2 +b^2 = c^2, where a and b are the sides and c is the hypotenuse, we can rplace a and b with the side lengths and solve for c.
7^2 + 3^2 = c^2
49 + 9 = c^2
58 = c^2
c = √58 ( Length of hypotenuse)
If you need it as a decimal: √58 = 7.615773 ( Round answer as needed.)