Since you listed this as basic chemistry I'm going to assume that you're using the simplified equation

ΔT_f = -K_f*C_m*i

You've got values for

ΔT_f = -2.5°C

K_f = 1.853 (K*kg)/mol

i = 1 because it is a non-electrolyte, there's no ionization

The only other trick would be to remember that molal concentration (C_m) is moles of solute per kg of solvent, and then that moles of solute is equal to

5g solute*(1 mol/x g solute).

so you end up with

2.5 = 1.853*[(5/x)/(0.025 kg)]

Just solve for x.