find

let x denotes the price and y denots the amount

so we have to find the straight line which passes through (14,980) (16,1220)

the line will be (y-980) = (1220-980) / (16-14)   (x-14)  =>y-980 = 120(x-14)

now x is given as 17. put in the equation of line

y =980 + 120 (17-14) = 1340

so he call sell 1340 litre of milk at the price of Rs. 17.