Let's call $r_A$ the rate at which printer A works. By definition, $rate = \frac{number of tasks }{ time}$ to complete them.
$r_A=\frac{300}{6}$
In 7 minutes, A produces $7 * r_A= 7 * \frac{300}{6 }= 350$

Now we do the same for B.
$r_B=\frac{300}{4}$
In 5 minutes, B produces $5 * r_B = 5*\frac{300}{4}=375.$

Thus B produces $375 - 350 = 25$more copies