How do you find the utility function and marginal utility in economics?

Here is some extra information, number of item a 0,1,2,3,4,5 price is $2.00. Item number b, 0,1,2,3,4,5 price is $1.00 and you have only $6.00 to spend. What is the utility maximizing combo for these two items. Please explain in detail have been going over but so lost!!!