You and your friend spent a total of $15 for lunch. your friend’s lunch cost $3 more than your lunch cost. how much did you spend for lunch?

The Answer is 6 Dollars

Well here there are two parts You and your friend spent 15 dollars your friends lunch cost 3 more than yours 

6=x is the amount you spent on lunch.
Your friend spent $3 more so add $3 to the amount you spent to get...
$9 spent by your friend. 
You=$6
Friend=$9
Total=$15
To prove the solution is valid, plug 6 in for x. 
15=3+2(6)
15=3+12
15=15 thus proving the solution is valid.

To solve for this, we will create a system of equations.
x + y = 15
x = y + 3
Let's try numbers that add up to 15.
I will try 6 and 9, 9 for x, 6 for y.
6 + 9 = 15
9 = 6 + 3
These system of equations works!
Since x is your friend's lunch, y is yours.
You spent $6.
I hope this helps!