#4x-63=9x+57#
We want to get #x# on one side, so we can either remove the #x# on the left side or the #x# on the right. To cancel out the #x# on the left, we need to subtract #4x# from both sides, or alternatively to cancel out the x on the left we subtract #9x# from both sides. For ease I subtracted #4x# from both sides.
#4x-63quadcolor(red)(-quad4x)=9x+57quadcolor(red)(-quad4x)#
#-63=5x+57#
Now we want to get #x# by itself, so we start by subtracting #57# from both sides:
#-63quadcolor(red)(-quad57)=5x+57quadcolor(red)(-quad57)#
#-120=5x#
Divide both sides by 5 to get x completely by itself:
#-120/color(red)5=(5x)/color(red)5#
#-24=x#
#x=-24#