Jacob, kylie, and manuel each bought a sandwich and drink. the total bill to $21, and each drink cost $2. of each sandwich cost the same amount, what is the cost of one sandwich?

So each individual bought one drink and one sandwich. The equation to find the cost is 21 = 3x + 6
Subtract 6 from both sides 
15 = 3x
Divide 3 from both sides 
x = 5
The price of each sandwich was $dollars.

Think of it, 5 + 5 + 5 = 15 (sandwiches)
15 + 2 + 2 + 2 = 21 (Drinks and the total sandwich amount)

Hope this makes sense, ask if you need more help! :)