6 7/8 + 3/2 as a fraction

How do i simply this expression (quadratic formula basis) on a ti-84 or normal calculator?

Math i have no idea how to do a and b

Becca wants to mix fruit juice and soda to make a punch. she can buy fruit juice for $3 a gallon and soda for $4 a gallon. if she wants to make 28 gallons of punch at a cost of $3.25 a gallon, how many gallons of fruit juice and how many gallons of soda should she buy?