Two sides of a paralellogram measure 7.0cm and 9.0cm. THe longer diagonal is 12 cm long. Calculate all the interior angles , to the nearest degree, of the paralellogram.
Do you know the Law of Cosines?
$\displaystyle c^2 = a^2 + b^2 - 2ab\cos(\theta)$, so
$\displaystyle 12^2 = 7^2 + 9^2 - 2(7)(9)cos(\theta)$.
Solve that for $\displaystyle \theta$, and you have one angle, and its opposite. The other two angles are supplements of that angle ($\displaystyle 180 - \theta$).