I believe a preposition and article were lost in the text of the question.
What is the length of a diagonal of a rectangular picture whose sides are 3 inches and 4 inches?
One method of finding the answer is by the use of the formula of the Pythagorean theorem. The equation is c = (a^2 + b^2)^(1/2), where a and b represent the lengths of the sides of the rectangular picture, and c, the length of the diagonal.
c = (3^2 + 4^2)^(1/2)
= (9 + 16)^(1/2)
= (25)^(1/2)
= 5 [inches].