A solution of 7.50 mg of a small protein in 5.00 mL aqueous solution has an osmotic pressure of 6.850 torr at 23.1°C.?

Osmotic pressure, Π = 6.850/760 atm
Molarity, M = ? mol/L
Gas constant, R = 0.08206 L atm / (mol K)
Temperature, T = (273.2 + 23.1) K = 296.3 K

Π = MRT
6.850/760 = M × 0.08206 × 296.3
Molarity, M = (6.850/760) / (0.08206 × 296.3) mol/L = 3.707× 10⁻⁴ mol/L

Molar mass of the protein = (7.50/1000 g) × (5.00/1000 L) / (3.707 × 10⁻⁴ mol/L) = 4050 g/mol
(to 3 sig. fig.)

Just to add a bit to sioieng's comment:

The van 't Hoff factor for the protein is 1. This is because the protein does not ionize in solution. If you asked a similar question (determine molar mass) of an substance that does ionize, you'll need to include a non-zero van 't Hoff factor.