Check the picture below.
and if you check your Unit Circle, you'd notice that those angles have an specific value.
[tex]\cfrac{15}{sin(60^o)}~~ = ~~\cfrac{x}{sin(45^o)}\implies \cfrac{15}{~~ \frac{\sqrt{3}}{2}~~}~~ = ~~\cfrac{x}{~~ \frac{\sqrt{2}}{2}~~}\implies \cfrac{15}{1}\cdot \cfrac{2}{\sqrt{3}}~~ = ~~\cfrac{x}{1}\cdot \cfrac{2}{\sqrt{2}}[/tex]
[tex]\cfrac{30}{\sqrt{3}}=\cfrac{2x}{\sqrt{2}}\implies \cfrac{30\sqrt{2}}{\sqrt{3}}=2x\implies \cfrac{30\sqrt{2}}{2\sqrt{3}}=x\implies \cfrac{15\sqrt{2}}{\sqrt{3}}=x \\\\[-0.35em] ~\dotfill\\\\ \stackrel{\textit{rationalizing the denominator}}{\cfrac{15\sqrt{2}}{\sqrt{3}}\cdot \cfrac{\sqrt{3}}{\sqrt{3}}\implies \cfrac{15\sqrt{6}}{3}}\implies \boxed{5\sqrt{6}=x}[/tex]
