This is the graph of what you want at the top.
You can also do it like this:
If we have f(x) = ax + b, then depend on the value of constant "b" we will find the place that our graph has cut the "y" scale as you see in picture.
So In f(x) = -x + 6 we know, f(x) = y
and -x means coordinate of x thats negative and 6 means the place that our graph has cut the y scale.
.::good luck::.
