You have to do Pythagorean theorem:
[tex]a^{2} +b^{2} = c^{2}[/tex]
"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:
[tex]7^{2} + x^{2} = 19^{2}[/tex]
[tex]49 +x^{2} = 361[/tex]
Isolate x by subtracting 49 to both sides:
[tex](49-49) + x^{2} = 361 - 49[/tex]
[tex]x^{2} = 312[/tex]
To solve for x you have to take the square root to both sides of the equation:
[tex]\sqrt{x^{2} } = \sqrt{312}[/tex]
x = 17.6635
x = 17.7
Hope this helped!
