Find the cost of 3 notebooks and 1 magazine in terms of y, if the cost of 1 notebook is 2y and that of 1 magazine is (y+3) ?

Answer 1

that would be

3(2y) + 1(y+3) = 6y+y+3 = 7y+3

Answer 2

This is how you solve the problem.
3n+1n=0
3(2y)+1(y+3)=0
6y+y+3=0
7y+3=0
7y+3-3=0-3
7y=-3
7y/7=-3/7
y=-3/7