Answer:
To do this, you need to multiply out the expressions. This is a bit tedious, but remember like FOIL for binomials, for these trinomials you must multiply each term. If you need a step-by-step, I'd be happy to provide it. Let me know.
Once you have simplified the expression, you get
-x-9/2x-4
But, the problem stipulates that a must equal 1. We can equivalently factor out the negative sign and put it on the denominator with no change to write
x+9/-(2x-4) = x+9/-2x+4
So, seeing where each coefficient corresponds between the two expressions, you get a = 1, b = 9, c = –2, and d = 4.
3/4 Of X(Capacity) = 18 Gallons
X = 18*4/3
= 6*4
= 24 Gallons
Capacity of Tank = 24Gallons
translates to 
