What is the y=mx+b of this?

What is the y-intercept of the line given by the equation

Water flows at a steady rate from a tap. its takes 40 seconds to fill a 4 litre watering can from the tap. the rate at which water flows from the tap is halved. complete: 4 litres into cm3

Which expression do you get when you eliminate the negative exponents of 4a^2 b^216a^-3 b