Concentration: 2.5% w/v
Concentration w/v % = [mass of solute / volume of solution ] * 100
=> mass of solute = concentration w/v * volume of solution / 100
=> mass of solute = 2.5 * 750 ml / 100 = 18.75 grams
Also, using proportions you get to the same result:
2.5% w/v => 2.5 grams of NaOH / 100 ml of water
=> 2.5 grams of NaOH / 100 ml water = x grams of NaOH / 750 ml water
=> x grams of NaOH = 750 ml water * 2.5 grams of NaOH / 100 ml water
=> x = 18.75 grams of NaOH
Answer = 18.75 grams of NaOH
