Q: Find the number of ordered pairs $(x,y)$ such that $5\cdot{}^xC_y=3\cdot{}^7C_3$ Find the number of ordered pairs $(x,y)$ such that  $5\cdot{}^xC_y=3\cdot{}^7C_3$
I am very new to number theory and combinatorics, so I could not proceed much. I tried to break ${} ^7C_3$ in the factorial form and then tried to simply, but it was not of any help.    
So, any hint or answer will be appreciated.

A: \begin{align}
5{x \choose y} & = 3{7 \choose 3} \\
& = 3 \cdot \frac{7!}{3!(7-4)!} \\
& = 3 \cdot \frac{7 \cdot 6 \cdot 5 \cdot 4!}{3!\cdot4!} \\
& = 3\cdot \frac{7\cdot 6\cdot 5}{3!} \\
& = 7 \cdot 5 \cdot 3 \\
& = 105 \\
\implies {x \choose y} & = 21 \\
\end{align}
We know that $x$ is at most $21$, because the values are increasing as $x$ increases. Obvious solutions are $(21,1)$ and $(21,20)$. Looking at Pascal's Triangle, we see that $(7,2)$ and $(7,5)$ are answers as well.
Hence there are $4$ solutions which are $(7,2), (7,5), (21,1),(21,20)$.
