Hey there!
27 tells us the sum of protons and neutrons is 27.
Al tells us we have 13 protons.
3+ tells us that there are 3 less electrons than protons.
13 + n = 27
neutrons = 14
13 - 3 = 10 electrons
27Al3+ has 13 protons, 14 neutrons, and 10 electrons.
Hope this helps!