Based on the Pythagorean Theorem, if you have a right triangle with sides \( m \) and \( n \) and a hypotenuse \( a \), the relationship of the sides is given by the equation:
\[ m^2 + n^2 = a^2 \]
This equation states that the sum of the squares of the lengths of the two legs (sides \( m \) and \( n \)) of the triangle is equal to the square of the length of the hypotenuse (side \( a \)).
So, the answer to your question is:
\[ m^2 + n^2 = a^2 \]