So he bought 12 CDs and DVDs (in total), CDs + DVDs = 12
Each cd is 2 and each cd is 3
Total spent = 31 (2*x + 3*y)
I call x cds and y dvds
x+y = 12
2x+3y = 31
Solve for y
Move x to the right with changed sign
y = 12-x (DVDs, you can also write x = 12-y)
And substitute 12-y in the equation
2x + 3(12-x) = 31
2x + 36 - 3x = 31
2x - 3x = 31 - 36
-x = -5
x = 5 (CDs)
To find DVDs let's rewrite y = 12-x
y = 12-5 = 7 (DVDs)
