According to one economic model, the demand for gasoline is a linear function of price. if the price of gasoline is p # $3.10 per gallon, the quantity demanded in a fixed period of time is q - 65 gallons. if the price is $3.50 per gallon, the quantity of gasoline demanded is 45 gallons for that period.
a) find a formula for q (demand) in terms of p (price).
b) according to this model, at what price is the gas so expensive that there is no demand.

Your first line calculation is already true but reversed. The demand will decrease (that mean the formula would be minus) by 20 gallons for every $0.4 price increase, which means 50 gallons/$. To find the formula you need to insert one sample of either the first equation (65gallon and $3.1) or 2nd equation (45 gallons and $3.5). The formula for demand should be:

q= C - 50p
65 gallon= C- 3.1 * 50 
C= 65 gallon + 155 gallon= 220 gallon

The formula would be:
q= 220- 50p

So the second question can be solved by putting 0 on the Q. It would be:
q= 220- 50p
0=220-50p
50p=220
p= $4.4