A store mixes red fescue worth $12 per pound and chewings fescue worth $15 per pound. The mixture it to sell for $14 per pound. Find how much of each should be used to make a 228 pound mixture

Let the amount of the red fescue be 'a' and that of the other be 'b'

The total mixture will have a mass of a + b = 228

Then, their price will be related to their mass.

1 pound -> $12
a pounds -> 12a

So, b pounds -> 15b

So that:
12a + 15b = 14

You have two equations with two unknowns. Find a and b using simultaneous equations :)