arrange 2^75, 3^50, 7^25
Compare $\displaystyle 2^3$, $\displaystyle 3^2$ and 7 (put them in order of size), then raise to the power of 25.
For example, $\displaystyle 2^3=8$ and $\displaystyle 3^2=9$, so $\displaystyle 2^3<3^2$; therefore
$\displaystyle 2^{75}\ =\ (2^3)^{25}\ <\ (3^2)^{25}\ =\ 3^{50}$
Do the same with the 7 as well.