Answer:
a) Demand function: 
b) The rebate should be of $200, so the sale price becomes $300 per unit.
c) The rebate should be of $150, so the sale price becomes $350 per unit.
Step-by-step explanation:
a) In this case, we have a known point of the demand function (1000 units sold at $500), and the slope of a linear function (increase by 100 units fora decrease in $10).
We can express the demand function (linear) as:

To calculate the slope m, we use:

To calculate b, we use the known point and the calculated slope:

Then the demand function is:

b) The revenue can be expressed as:

To maximize, we can derive and equal to zero

The rebate should be of $200, so the sale price becomes $300 per unit.
c) If we take into account the cost, we have that

To maximize, we can derive and equal to zero

The rebate should be of $150, so the sale price becomes $350 per unit.