a) The quantity q, in other words metres of fabric, is 10000m when the price is $20 per metre.
f'(20) = -350 means that at the point where the price is $20/m the quantity is decreasing at the rate of 350 m per unit price change. In other words, if the unit price decreases to, say, $19/m, then the quantity increases. If the unit price increases then the quantity decreases.
b) The revenue is how much the manufacturer will get from the sale of q metres of fabric when the cost to the customer is $20/m. 20 * 10000=200000 is the revenue from the sale. R'(20)=20f'(20)=20 * -350=-7000.