The perimeter of any triangle is simply the sum of the lengths of the three sides; so, if p is the perimeter and the three sides are a, b, and c, then
p = a + b + c
Then, by the Pythagorean theorem, if c is the hypoteneuse (the side opposite the right angle), then
a^2 + b^2 = c^2
So, just plug in the squares of the two lengths you already have, solve for the remaining side, and take the square root of that. For example, if you already know the hypoteneuse is 5, and that one other side is 3, then
To find the perimeter of a right triangle when you have the length of two of the sides, you can use the Pythagorean theorem to first find the length of the third side. The Pythagorean theorem is a^2 + b^2 = c^2, in which c is the hypotenuse, the longest side, or the side opposite the right angle. Once you find what c is equal to, you can add the lengths of all three of the sides, which will equal the perimeter, or the total of all the lengths of the sides.