The legs of a right triangle are four units and 7 units what is the length of the hypotenuse

This is where Pythagorean's Theorem comes into play. a^2+b^2=c^2
So take 4^2 + 7^2 (which is 16+49), and your result is 65. Take the square root of 65 and you get 8.06 (8.1 rounded up)