Boyle's law gives the relationship between volume and pressure of a gas. It states that pressure is inversely proportional to volume of gas when the temperature is constant.
PV = k
where P - pressure, V - volume and k is constant
P1V1 = P2V2
where P1 and V1 are parameters for the first instance and P2 and V2 are the parameters for the second instance.
substituting values in the equation,
300 kPa x 0.4 L = P x 1.750 L
P = 68.6 kPa
new pressure is 68.6 kPa