This the equation you will need to use (the Pythagorean Theorem)
[tex]a {}^{2}  + b {}^{2}  = c[/tex]
when you put the numbers in for the variables given it will look like this
[tex]( {3})^{2}  + ( {4})^{2}  = c[/tex]
3 squared equals 9 and 4 squared equals 16:
[tex]9 + 16 = 25[/tex]
so side c is 25 units