Cosine Rule (missing angle)
\begin{align} a^2=b^2+c^2-2bcCosA \\ \\ \therefore \quad CosA= \frac{b^2+c^2-a^2}{2bc} \end{align}
Find x° (to 2 d.p.)
12
17
7
x°
\begin{align} cos\ x & = \frac{12^2+7^2-17^2}{2(12)(7)} \\ & = -0.57\\ \\ So \quad cos^{-1}(-0.57) & = x \\ \implies \ x & = 124.75^{\circ} \end{align}