Melody is exactly correct.......we would need more info to obtain a specific answer......
However........I'm going to change your question slightly to this one:
What could be the length of side c of a right triangle when a and b equal 3 and 4??
Well, by something known as the "triangle inequality," the remaining side is either greater than 1, or less than 7.
Then....either 4 is the longest side (the hypotenuse), or it isn't.
Let's suppose that it is......then, by the Pythagorean Theorem, the remaining side is...... SQRT(4^2 - 3^2) = SQRT(16 - 9) = SQRT(7) ........ which is greater than 1
So, if we're not too picky about whether a side is an integer or not, this could be one solution.
Now, let's suppose that 4 isn't the longest side. Then, again by the "P" Theorem, the remaining side is..... SQRT(3^2 + 4^2) = SQRT(9 + 16) = SQRT(25) = 5.........which is less than 7
So, we have two possible answers.......one right triangle has sides of SQRT(7), 3 and 4, where 4 is the hypotenuse. And the other triangle has sides of 3, 4 and 5, where 5 is the hypotenuse!!
I'm not sure which applies to your particular situation, but exploring possibilities is sometimes more interesting than figuring out "exact" things....
Hope this helps..