The idea here is that you need to assume that the temperature of the balloon remains constant because this will allow you to use Boyle's law to calculate the pressure of the balloon at its new volume.
So, Boyle's Law states that when the temperature and number of moles of gas, i.e. the amount of gas, are being kept constant, the pressure and the volume of the gas have an inverse relationship
#color(blue)(ul(color(black)(P_1V_1 = P_2V_2)))#
#P_1#and #V_1#represent the pressure and volume of the gas at an initial state
#P_2#and #V_2#represent the pressure and volume of the gas at a final state
Now, the volume of the gas increases at it moves away from sea level, which implies that its pressure decreases.
Rearrange the equation to solve for
#P_1V_1 = P_2V_2 implies P_2 = V_1/V_2 * P_1#
The pressure at sea level is defined as
#P_2 = (90.0 color(red)(cancel(color(black)("L"))))/(175.0color(red)(cancel(color(black)("L")))) * "1 atm" = color(darkgreen)(ul(color(black)("0.514 atm")))#
The answer is rounded to three sig figs, the number of sig figs you have for the initial volume of the gas.
As predicted, the volume of the gas increased because the pressure decreased.