Divide both side by $50$ first.
Then undo the exponent by raising both sides to the $3/2$ power

Combine the terms on the left side into one logarithm.

$\log_3\left(\left(x+1\right)\left(x\right)\right)=\log_3\left(12\right)$

Divide by $50$ first.
Then take the log of both sides, bringing the $x$ down in front.
Solve for $x$.

This problem is similar to part (b), except the subtraction indicates that you need to divide the arguments when creating the single logarithm instead of multiplying them.