What mass of sucrose c12h22o11 is needed to make 300 ml of a 0.50m solution?

Use the concentration to obtain the moles. I am assuming you mean to write capital M. because little m means molality.

So, first convert the ml into Liters and then into moles, then moles to grams using the molar mass (just adding the values of each atom from the periodic table.)

Molar mass = 12 (12.0) + 22 (1.01) + 11 (16.0) = 342 grams/mole

300 ml (1 liter / 1000 mL) x (0.50 moles / 1 Liter) x (342 grams / 1 mole) = 51.3 grams