First lets convert 750 ml into number of cups.

250 ml = 1 cup

1 ml = 1/250 cup

750 ml = (1/250) * 750 = 3 cups

Quantity of flour required to make 30 muffins = 750 ml = 3 cups

Quantity of flour required to make 1 muffin = 3/30 cups

Quantity of flour required to make 90 muffins = (3/30) * 90 = 9 cups

A recipe calls for 750 milliliters of flour to make 30 carrot muffins. 

Assume 1 cup = 250 ml .

Round your answer to one decimal place.

9.0 cups of flour are needed to make 90 muffins.

just multiply by 3, surely you can do that. Then divide by 250 to convert to cups.