“Bananas per strawberry” means bananas/strawberry=8.4/6=1.4 which you got as the answer.
Perhaps they were looking for whole numbers. 1.4=1⅖=7/5 which is 7 bananas for every 5 strawberries.
Read the question again and see if you have the correct interpretation. As a ratio the answer would be 7:5. As a fraction it would be 7/12 of a mixture are bananas and 5/12 are strawberries.