As this is a right triangle, we can use the pythagoras theorem, which states that if a and b are the shorter and c the longest side of the right triangle: a^2 + b^2 = c^2.
Here we are given the longer side c=34and one of the shorter sides (does not matter if we use a or b, let us say b=30).
Then a^2 + 30^2 = 34^2.
Therefore a^2 = 34^2 -30^2
Hence a = sqrt(34^2 -30^2)
(Just plug it into a calculator <3)