Follow the 
a² + b² = c²
formula, in which c² is the longest side (hypotenuse).
Let ? = x
x² + 17² = 19²
Simplify:
x² + 289 = 361
Isolate the x, subtract 289 from both sides
x² + 289 (-289) = 361 (-289)
x² = 361 (-289)
x² = 72
finally, to find the side, square root both sides to isolate the x
√x² = √72
x = √72
Simplify √72
                         √72
                         |     |
                         8    9
                       |   |  |  | 
                       4 2  3 3 
√3 x √3 = 3
√4 = 2
3 x 2 = 6
6√2
6√2 should be your answer
hope this helps