As you know, pressure and volume have an inverse relationship when number of moles and temperature are kept constant - this is known as Boyle's Law.
That happens because gas pressure is caused by the collisions between the gas molecules and the walls of the container.
If temperature, which is actually a measure of the average kinetic energy of the gas molecules, is kept constant, increasing the volume of the container will result in less frequent collisions.
Simply put, the molecules will have more room to move about in, so they won't hit the walls of the container as often
Likewise, decreasing the volume means giving the molecules less room to move in. As a result, they'll hit the walls of the container more often
Mathematically, this is expressed as
#color(blue)(P_1V_1 = P_2V_2)" "#, where
Notice that in your case, the pressure of the gas increased from
Plug in your values and solve for
#P_1V_1 = P_2V_2 implies V_2 = P_1/P_2 * V_1#
#V_2 = (3.0 color(red)(cancel(color(black)("atm"))))/(7.0color(red)(cancel(color(black)("atm")))) * "5.0 L" = color(green)("2.1 L")#
The answer is rounded to two sig figs.