Answer:
p = -0.05m + 22
Explanation:
As given in question,
The relation between the price and demand can be given by,
where is p is price and x is demand.
Since in order to sell 300 cards the price has to be fix at $7.
So,
  7 = 300m+b   (1)
And for every additional card they need to drop the price by $0.05.
So,
7-0.05 = 301m+b 
=>6.95 = 301m + b  (2)
Subtracting equation (1) from equation (2), we will have
6.95 - 7 = 301m - 300m
=>m = -0.05 
By putting the value of m in equation (1), we get
 7 = 300.(-0.05)+b
 => b = 22
So, the linear equation that show the relation between price and demand is given by 
p = -0.05m + 22