Justify each step. 2x-5=-20 2x=-15 x=-15/2

This is a linear equation, and it is solved for finding the value of x.
2x - 5 = -20
now simple number -5 goes to the other side of equation and turns into +5, then the equation becomes
2x = -20 +5
now we get 2x = -15
because -20 + 5 = -15, greater number is 20 then 5 and 20 is with - sign.
the next step is x = -15/2
in 2x, 2 is multiplying with x when it goes to the other side of equation 2 will be divided with -15, that's why we get
x = -15/2
which is the value of x

The discount is 8.70 . you dont tell us the percent of tax added so there is no way to find out, unless i'm not seeing anything. without tax its 37.70