Since there is no info on how many salads were sold all together it would have to be 12. That is all the info you could give us..sorry if that doesnt help :(
Slope = (-14-1)/(7-1) = -15/6
y = mx + c
y = -15/6 x + c
at (1,1)
1 =-15/6(1) + c
c = 1 + 15/6 = 21/6
y = -15/6 x + 21/6 or
6y = -15x + 21