Use the Pythagorean theorem /look at the picture/
? = x
[tex]1.\\x^2+13^2=28^2\\x^2+169=784\ \ \ |subtract\ 169\ from\ both\ sides\\x^2=615\\\boxed{x=\sqrt{615}}\\\\3.\\x^2+10^2=28^2\\x^2+100=784\ \ \ \ |subtract\ 100\ from\ both\ sides\\x^2=684\\x=\sqrt{684}\\x=\sqrt{36\cdot19}\\x=\sqrt{36}\cdot\sqrt{19}\\\boxed{x=6\sqrt{19}} [/tex]
