Answer
$C = 95.7˚$
Work Step by Step
$cos(C) = \frac{a^{2} + b^{2} - c^{2}}{2ab}$
$cos(C) = \frac{(5)^{2} + (7)^{2} - 9^{2}}{2(5)(7)}$
$cos(C) = \frac{25 + 49 - 81}{70}$
$cos(C) = \frac{74 - 81}{70}$
$cos(C) = \frac{-7}{70}$
$cos(C) = \frac{-1}{10}$
by GDC / calculator
$C = 95.739...˚$
$C = 95.7˚$
