First use pythagorean theorem to find the missing side of the triangle..
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A^2 + b^2 = c^2            Pythagorean Theorem
3^2 + 4^2 = c^2            Substitute for known variables
√3^2+4^2 = √c^2          Take the square root from each side
5 = c                             Simplify

So, the length of the missing side is 5. I hope this helps!