You can 
put this solution on YOUR website! .
Let x = the hotel charge before tax in the first city;
    y = that of the second city.
Then from the condition you have these two equations in two unknowns
       x  -     y = 500     (1)
    0.08x + 0.04y = 640.    (2)
To solve the system of equations, use the Elimination method.
For it, from equation (1) express  x = 500 + y  and substitute it into the second equation. You will get
    0.08*(500+y) + 0.04y = 640.
Simplify and solve for y
    40 + 0.08y + 0.04y = 640
    0.12y = 640 - 40 = 600
    y = 600/0.12 = 5000.
ANSWER.  In the second city hotel charged $5000;  in the second city  $500 dollars more, or  5500  dollars.
CHECK.   0.08*5500 + 0.04*5000 = 640  dollars.    ! Correct !