#y+7x = 9#

There are 2 variables in this equation, which means that you can't solve it and find ONE unique solution. There are infinitely many solutions.

"Solve for y" means write the equation as #y = ..........#

#y +7x -7x = 9 -7x" "larr# subtract #7x# from each side

#" "y = -7x+9" "larr# equation of a straight line

Now you have been given various values of #x#. Use each in turn in the equation and find the corresponding values of #y#

#" "y = -7color(magenta)(x)+9#

#color(magenta)(x =-1) rarr" " y = -7color(magenta)((-1)) +9#

#color(white)(xxxxxx.xxxxx)y = +7+9 = 16#

#color(magenta)(x =0) rarr" " y = -7color(magenta)((0)) +9#

#color(white)(xxxxxx..xxxx)y = 0+9 = 9#

#color(magenta)(x =4) rarr" " y = -7color(magenta)((4)) +9#

#color(white)(xxxxxx...xxxx)y = -28+9 = -19#

What you have actually found is three points, #(x, y)# on a line.

#(-1, 16)" "(0.9)" "(4, -19)#