Equivalent expressions SAT question

The correct answer is $6$. Applying the distributive property to the expression $r{y}^4(15y-9)$ yields $15r{y}^5-9r{y}^4$. Since $90y^5-54y^4$ is equivalent to $r{y}^4(15y-9)$, it follows that $90y^5-54y^4$ is also equivalent to $15r{y}^5-9r{y}^4$. Since these expressions are equivalent, it follows that corresponding coefficients are equivalent. Therefore, $90=15r$ and $-54=-9r$. Solving either of these equations for $r$ will yield the value of $r$. Dividing both sides of $90=15r$ by $15$ yields $6=r$. Therefore, the value of $r$ is $6$.