You need to use the Pythagorean theorem. [tex]{a}^{2} + {b}^{2} = {c}^{2} [/tex] The hypotenuse is "c" and each leg is either "a" or "b". [tex] {10}^{2} + { x}^{2} = {17}^{2} [/tex] [tex]100 + {x}^{2} = 289 \\ {x}^{2} = 189 \\ x = 13.7477270849[/tex]