Question 714517
y        = -5x
x + 0.8*y = -7.2

To solve this system you need to put both equations in one equation.

First equation: y =      -5x 
so 2*y =   2*(-5x)
and therefore 0.8*y = 0.8*(-5x) 
which we have in the second equation, so we can put 0.8*y in the second equation.

x + (0.8*(-5x)) = -7.2
x +       -4x   = -7.2
          -3x   = -7.2
            x   = -7.2/-3
            x   = 2.4

Put x in the first equation and you get the value of y.

y = -5x
y = -5*2.4
y = -12