Answer:
the missing side is a square root of 1475 or if you want a more rounded answer then it would be 38.4
Explanation:
Use the Pythagorean theorem a2+b2=c2
17 squared + b2 = 42 squared
289+b2=1764
subtract 1764 and 289 =
Square root 1475 your answer
more rounded answer
38.4