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.
Final answer: 75
Hope this helps.