How do I find the percent modeled by a grid?

How to do this , and can some one explain

Atriangular field has sides of lengths 21, 39, and 41 yd. find the largest angle. (round your answer to the nearest degree.)

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?