Concentration unit is moles/Liter.
So first we need to transform 63.0 g of phosphoric acid into moles. For this, we use the following equation:
mole = mass/molar mass
mole = 63.0/98.00
mole = 0.643 moles of H3PO4
Now, to find the concentration (molarity - M) we use the following equation:
M = moles/volume of solution
First, transform 250 mL into Liters. Just divide by 1000:
250 mL = 0.250 liters
M = 0.643/0.250
M = 2.57 mol/L
Answer: M = 2.57 mol/L