The shortage can be calculated as follows. Set the price ceiling price equal to the demand equation and equal to the supply equation and solve for Qd and Qs respectively.
|Demand||1 = 10 - 0.9Qd|
|Qd = 10|
|Supply||1 = 0.5 + 0.5Qs|
|Qs = 5.25|
Subtracting Qs from Qd, we have a shortage of 4.75 units.