Here is what the answer to your question is:
7x10x1=70
is 4.
Then, 7(-2) is -14.
Then, 4 - (-14) is (-18).
Finnaly, (-18) - 8 is
[-26]
Let us assume the total amount of meal charge paid by Michael without tax = x
Then
Amount paid to the waiter by Michael = 15x/100
Sales tax paid by Michael = 5x/100
Now we get the equation as
x + (5x/100) + (15x/100) = 18
(100x + 5x + 15x)/100 = 18
120x/100 = 18
120x = 1800
x = 1800/120
= 180/12
= 15 dollars
So the amount paid by Michael for the meal before tax and tip is $15. I hope the procedure is clear enough for you to understand.