Since you have not asked a specific question ill assume you are stuck at step 1.

Step 1:

The excess demand function is D(p) - S(p).

The Equilibrium price is the value p for which excess demand is zero.

So, to find an equilibrium see if:

30-2p = -10 + 3p has a solution

and see if

50 - ap = -10 +3p has a solution