$18.37 is the cost of just the dinner.
Add 6% of this as tax by either multiplying by dinner cost by 1.06 (to get dinner + tax)
For tip... If you calculate the 15% before, then you would add it together with the tax to get 21%, and then your answer would be calculated in the same way as just the tax.
If you tip the amount after tax, then take your answer for the (dinner + tax) part and use the same method, but with 15%.
(dinner + tax) x 1.15 = dinner, tax + tip
