Hey, I just got this problem wrong too a little while ago, but now understand the solution.
It's an ICE setup:
Initial:
PCl5: 4.750 M
PCl3: 0 M
Cl2: 0 M
Change:
PCl5: -1.896 M
PCl3: +1.896 M
Cl2: +1.896 M
Equilibrium:
PCl5: 4.750 - 1.896 = 2.854 M
PCl3: 1.896 M
Cl2: 1.896 M
PCl5 <---> PCl3 + Cl2
Kc = [PCl3][Cl2] / [PCl5]
Kc = (1.896)(1.896) / 2.854
Kc = (1.896)^2 / 2.854