Let the price of a bag of balloon=x and the price of a roll of crepe paper =y
5x+2y=38
12x+14y=128
These are the equation you get from the question
5x+2y=38
-5x -5x
2y=38-5x divide both side by 2
y=19-5/2x
12x+14y=128 plug in the y
12x+14(19-5/2x)=128
12x+266-35x=128
-23x+266=128
-266 -266
-23x=-138 divide both side by -23
x=6 this will be the price of a bag of balloon
5x+2y=38 plug in x
5(6)+2y=38
30+2y=38
-30 -30
2y=8 divide both side by 2
y=4 this will be the price of a roll of crepe paper