Here I would like to help you because I also had trouble with these yes someone helped you already but I would like to help you learn whats going on I'm assuming you have a basic foundation of these equations now this one is fairly simple because we have no more like terms to add
so when we are down to this simple for we take the four and do inverse operations which is pemdas just reversed and we add 4 to 31 because
when you think about it 7 x E equals the true number its just when this equation is complete we subtract 4 so when we add four to 31 and we get 35 now here we divide 35 by seven to see what E equals because seven times e = 35 when we divide we find e to = 5 and this makes sense let's check are solution now 7 x 5 = 35 35-4 = 31
and yes the solution is correct 31=31.