c=(5,-3)
xa+yb=(x,-2x)+(3y,y)=(x+3y,-2x+y)
则有
x+3y=5-->2x+6y=10 1
-2x+y=-32
1式+2式得
9y=7
y=7/93
3式代入x+3y=5得
x=8/3
所以
c=x*8/3+y*7/9