Man, these types of problems are given me a run for my money

$\displaystyle t2 - 7t + 16$

Find the instantaneous velocity at t = 4

I get this problem simplified down to

$\displaystyle \frac{x(x-7)-a(a-7)}{x-a}$

Which I know is right.

But can i cancel the denominator by making the equation $\displaystyle \frac{(x-7)(a-7)}{1}$ then plugging in 4 and getting 6