#5x = 15# Added the #-2x# on the right side to the #3x#
#x = 3# Now divide #5# from the left side to the #15# on the right side to isolate #x#.
Leaving you with the answer of being #3#. You can also check your work by plugging the number back into #x# with the starting equation. They should be equal if your answer is correct.