I am guessing that the 91 points are the 65%. I do not completely understand the question but this is my best analysis. So from my perception of the question is that the 91 points are equal 65%. This equation is just saying:
91 = 0.65x
So all you have to do is divide each side by 0.65:
x=140.
The number of points possible on the test is 140. Have a splendid day. ;)
(here is the process I did)
91=0.65x
Step 1: Flip the equation.
0.65x=91
Step 2: Divide both sides by 0.65.
0.65x/0.65 = 91/0.65x = 140
Answer: x = 140