We have to start by converting our grams to milligrams to have an equal equations.
.75g = 750mg.
We'll set up the equation of D (desired dose)/H (dose you have) x V (how many mL's) 
D = 750mg 
H = 250mg
V = 5mL 
750 ÷ 250 = 3
3 x 5 = 15.
Therefore, you'll be preparing 15mL's