Answer :
4x + 89.95 = 628.35 <== ur equation
4x = 628.35 - 89.95
4x = 538.40
x = 538.40 / 4
x = 134.60 <== the cost for each cavity filling
4x = 628.35 - 89.95
4x = 538.40
x = 538.40 / 4
x = 134.60 <== the cost for each cavity filling