help with math please its a word problem

Question

The calorie count of a serving of food can be computed based on its composition of carbohydrate, fat, and protein. The calorie count C for a serving of food can be computed using the formula C=4h+9f+4p , where h is the number of grams of carbohydrate contained in the serving, f is the number of grams of fat contained in the serving, and p is the number of grams of protein contained in the serving.
A serving of raisins contains 115 calories and 21 grams of carbohydrate. If raisins are a fat-free food, how much protein is provided by this serving of raisins?

Answer 1

C= 4h + 9f + 4p

 

C = 115

h = 21

f = 0 (i.e. fat-free)

p = ?

 

115 = 4(21) + 4p

 

Can you solve for p and finish from here?