when solving sales tax it's pretty simple because all you do is add everything for a subtotal and multiply your sub total by the tax so i'll use the first as an example your subtotal is 19.96 x0.05 to get 0.99 <-- your sales tax and your final total with tax to be 20.96
The slope, if increased, would make the line y=2x-7 steeper. If you were to increase the y intercept, you would just move the line, not tilt it. Suggest changing it to 4x-7 = y or 8x-7= y, if you increase the slope, it'll make it steeper.
Let 
. Then
lies in the second quadrant, so
So we have
and the fourth roots of are
where 
. In particular, they are
