Solution: Buy X New Year cards, that is, buy 14-X postcards.
0.35X+0.25( 14-X)=4
0.35X+3.5-0.25X=4
0. 1X+3.5=4
0. 1X=4-3.5
0. 1X=0.5
X=0.5/0. 1
X=5
14-5=9 (sheets)
I bought five New Year cards and nine postcards.