How would I use Ratios and Proportions in Algebra to solve this equation?

the change is not proportional, but it is linear
so you need to get a linear equation to solve this problem, not a proportional equation

to get the slope, you can use the formula
m = (y2-y1)/(x2-x1)

if you want to plot price on y axis, then y2 = $100 and y1 = $80
and number of units = 6.7 = x1 and x2 = 10.2
so calculate m

now write the linear equation governing the number of units sold and the price of units
it comes out to be
y-y1 = m( x- x1)
put values of y1, m , x1 in the above equation and put y = 75
you can get value of x at y = 75 i.e. you get he number of units sold at the price of $75