The total amount that your friend spent is the sum of the cost for 3 bags of wild bird seed and the bird feeder. By letting x be the cost of each bag of wild bird seed, the total cost for purchasing 3 bags of this is 3x. The equation that may be used to determine x is letter "B. 3x + 18 = 45". Each bag costs $9.
Use equation (y2-y1)/(x2-x1)
(3-(-1))/(0-5)
4/-5 is the slope
