It must be the negative that's bothering you. Imagine yourself on a staircase. -10p means you are 10 steps from the top. +9p says go up 9 steps. How many steps are you from the top? One, of course. So because you are one step down that's -1p or just -p. So the left side of the equation is -p. But the equation now says -p=12. Read this as 1 step down is 12 inches, or centimetres, or whatever. So if one step down is 12 how many inches (or whatever) down? It's 12 inches down or -12. So p=-12. Is that a bit clearer?