Simultaneous equation of two Graph line equations.?

Hi, i have two graphs

One that is Price against Demand. this has the line equation of

P(price) = 21 - 3/2 d (demand)

the second graph is price against supply which has the line equation of

P = 2S ( supply) + -7

Now then what i need to know how to do is, how to find out the price at which SUpply and demand are the same, by using a Simultaneous equation. I already know its 9 by putting them on the same graph but i need to do it with a Simultaneous equation thanks.

the actual question is.

By solving a set of two simultaneous equations find the price at which supply is equal to demand. What is the demand at such a price?