You can use this equation by substituting:
P1V1 = P2V2
P2 = P1V1/V2 (when the final pressure is missing)
given your question then,
your V1 or your initial volume = is 6.00 L (it must be expressed in Liter)
your P1 or initial pressure = 1.01 atm
your V2 or final volume = 3.00 L
then substitute using the formula given.
P2 = V1P1 /V2
= (6.00L)(1.01 atm) / (3.00.L)
= 6.06 atm / 3.00
= 2.02 atm
/:. the pressure needed is 2.02 atm.