How calculate how many milliliters of glycerin (specific gravity=1.26) will have a mass of 0.75 lbs?

Specific gravity is the ratio between the density of the material to the density of water at 4 degrees celcius (density of water at this temperature is 1000 kg/m^3)

1.26 = density of glycerin / 1000
density of glycerin = 1260 kg/m^3

1 Kg is equivalent to 2.2 lbs, therefore:
0.75 lbs = 0.34 kg

density = mass/volume
1260 = 0.34/volume
volume = 0.34/1260 = 2.857 x 10^-4 m^3

now, 1 m^3 is equivalent to 1000 liters
2.857 x 10^-4 m^3 = 0.2857 liters

one liter is equivalent to 1000 ml
therefore,
0.2857 l = 285.7 ml