Write an equation that relates the quantities.
The monthly payment $p$ on a mortgage varies directly with the amount borrowed $B$. If the monthly payment on a 30 -year mortgage is $\$ 6.49$ for every $\$ 1000$ borrowed, find a linear equation that relates the monthly payment $p$ to the amount borrowed $B$ for a mortgage with these terms. Then find the monthly payment $p$ when the amount borrowed $B$ is $\$ 145,000$.