Use Pythagoras to get the height, the use it to find x: 26^2-11^2 =555 555^1/2(square root of 555) =23.55843798 ~you need this number left as it is for a more accurate answer. Then you do 42^2-23.55...^2(which we know is 555) Then whatever that equals you square root that Then you round up the answer to that to get the final answer.
I left the last part so you can do it and do some active learning, if you have more questions on this please feel free to ask, hope this helps