Math Forum :: View topic – System of Equation

I have one math question, and I do not know how to solve. Can someone give me pointer? Solve x1,x2,x3,x4,x5,x6,x7,x8,x9 in the following system: y1=x3*a2+x5*a3+x7*a4 y2=x1*a1+x6*a3+x8*a4 y3=x2*a1+x4*a2+x9*a4 y1, y2, y3 are given positive constants. a1,a2,a3,a4 are given positive constants. y1+y2+y3=a1+a2+a3+a4 (e.g y1=1,y2=6,y3=3, a1=2,a2=3,a3=4,a4=1) x1+x2=1 x3+x4=1 x5+x6=1 x7+x8+x9=1