The difficulty with this formula is how they define â€śslopeâ€ť. As fino (correctly) calculated it,

slope = \frac{\Delta P}{\Delta Q} = \frac{$8 - $10}{300\ widgets - 200\ widgets} = -$0.02/widget

However, that doesnâ€™t work in the formula that they gave you in their hint:

MR = P - (Q Ă— slope)

As fino pointed out, the correct formula (using slope calculated correctly) is:

MR = P + (Q Ă— slope)

In my 2019 copy of the Level I curriculum, this formula is shown in the reading **Topics in Demand and Supply Analysis**, Â§3.2.2.

It appears that the author of your question wanted you to use this formula for the slope:

slope = \left\lvert\frac{\Delta P}{\Delta Q}\right\lvert = \left\lvert\frac{$8 - $10}{300\ widgets - 200\ widgets}\right\lvert = $0.02/widget

In that case, their formula is correct.

Note that, in either case, youâ€™re simply using the formula given; you donâ€™t need calculus.

Where did you get this question, if I may ask?