So problem says that 100 is deposit and 125 is withdraw.
So you have equation b = 100 + d - w. You have given information d = 100 and w=125, so all you have to do is to plug in values. You replace d to 100 and w to 125.
b = 100 + d - w b = 100 + (100) - (125) b = 75
So when d = 100 and w=125, the balance would be 75.
You know that d = 100 and w = 125, so you plug that into the equation and solve as shown: b= 100 + d - w = 100 + 100 - 125 = 200 - 125 = $75 You now have $75 left as balance.