45^2 &amp; = 28^2 + 49^2 - 2(28)(49) \cos M &amp;&amp; \text{Law of Cosines} \\ 2025 &amp; = 784 + 2401 - 2(28)(49) \cos M &amp;&amp; \text{Simplify squares} \\ 2025 &amp; = 784 + 2401 - 2744 \cos M &amp;&amp; \text{Multiply} \\ 2025 &amp; = 3185 - 2744 \cos M &amp;&amp; \text{Add} \\ -1160 &amp; = -2744 \cos M &amp;&amp; \text{Subtract}\ 3185 \\0.422740525 &amp; = \cos M &amp;&amp; \text{Divide by}\ -2744 \\65^\circ &amp; \approx M &amp;&amp; \cos^{-1} \ (0.422740525) 