[tex]\bf \begin{cases}
x=5cos(t)\implies \frac{x}{5}=cos(t)\\\\
cos^{-1}\left( \frac{x}{5} \right)=cos^{-1}[cos(t)]\\\\ cos^{-1}\left( \frac{x}{5} \right)=\boxed{t}\\
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y=5sin(t)\implies y=5sin\left( \boxed{cos^{-1}\left( \frac{x}{5} \right)} \right)
\end{cases}[/tex]
just a matter of substitution
