I am trying to find the residual of these 2 data sets

the asked question is

iii) Calculate the residual when the dietary intake is 20.4g.

Cholesterol level against Dietary Fibre intake per day 1000 900 800 700 600 Cholestrol level (mg/dL) 500 400 y = 3.874x + 191.75 300 200 100 0 0 5 10 15 20 25 30 35 40 Average Dietary Fibre intake per day (g)Cholesterol level against Dietary Fat inatke per day 1000 900 800 700 y = 2.74x + 31.001 R2 = 0.4945....... 600 Cholestrol level (mg/dL) 500 400 Series "Cholesterol level (mg/dL)" Point "13 (131.6, 550.5) 300 200 100 0 O 50 100 150 200 250 Average Dietary Fat intake per day (g)