y = 13.61x + 150.87
where:
y: Calories
x: fat (g)
To estimate the amount of fat if you consume 721 calories, you have to replace y = 721 into the equation and solve for x.
721 = 13.61x + 150.87
721 - 150.87 = 13.61x
570.13 = 13.61x
570.13/13.61 = x
42 grams = x