$40+$35=$75
75/100*8.75=6.5625
6.5625+75=81.5625
approximately equal to $81.56

.................................
General equation: y = mx + c
where m is the slope.
So if there’s a negative sign on mx, it is a negative slope and vice versa.
Start with

We observe that both fractions are not defined if
. So, we will assume
.
We multiply both numerator and denominator of the first fraction by 3 and we sum the two fractions:

We multiply both sides by
:

We move everything to one side and solve the quadratic equation:

We check the solution:

which is true