Question

Solve by cross multiplication 3x+y-11=0;x-y+1=0

Answer 1

3x+y-11=0
x-y+1=0

now, 

3x+y-11=x-y+1

transpose x to LHS

3x-x+y-11= -y+1

=> 2x +y-11= -y+1

transpose -y to LHS and -11 to RHS

2x+y+y= +1+11

2x+2y = 12 

2(x+y) = 12

x+y = 12/2 

=> x+y = 6

here we cannot find the value of x and y unless anyone of then is given....

Answer 2

given,
equations are
3x + y -11 = 0.......(1)
x - y + 1 = 0 .........(2)

     x                y            1
1          -11         3                 1       
-1          1          1                  -1
⇒  x/(1 + 11) = y/ (-11 +3)  = 1/( -3 +1)
⇒  x/12        = y/14  = 1/-2
⇒ -2x = 12    and -2y = 14
⇒ therefore x = -6 and y = -7