You have to do Pythagorean theorem:
"a" and "b" are the legs. In this case 7 is a leg and x is a leg
"c" is the hypotenuses, which is 19 here
so...
a = 7
b = x
c = 19
Plug the numbers above into the equation:
Isolate x by subtracting 49 to both sides:
To solve for x you have to take the square root to both sides of the equation:
x = 17.6635
x = 17.7
Hope this helped!