If the two legs are 7 units, this being a right angled triangle, by Pythagoras, the hypotenuse will be 7*sqrt(2) = 9.90 units long.Then the periimeter is 7+7+9.90 = 23.90 units.
leg+leg+hypotenuse
The legs are the two sides connected to the right angle(90). In a 45-45-90 triangle, the legs are the same length. The hypotenuse is the one not connected to the right angle (90)....
How you find the missing side depends on which side is missing. A square drawn on the longest side has the same area as the two squares drawn on the shorter sides put together. This is Pythagoras'...