A 20.0 % by mass solution of phosphoric acid (H3PO4) in water has a density of 1.114 g/mL at 20°C. What is the molarity of this solution?

Assume you have 1 L of the material (you can assume any volume you want, you'll get the same answer).

Using the density, the one L has a mass of 1114 g

20% by mass of phosphoric acid means the mass of phosphoric acid is 0.20 * 1114 g

Calculate the moles of phosphoric acid

Molarity, M, mole / L = moles of acid / volume or 1 L