That is true if, but only if, you label the measure of the hypotenuse of a
right triangle "c" and the measures of the legs of the right triangle "a" and
"b".  That is the usual way of labeling.  The Pythagorean theorem stated
in words is

In a right triangle the square of the measure of the side opposite the right
angle equals the sum of the squares of the measures of the sides that make the
right angle.

Edwin