Question 141884
Recall that a function assigns a y-value for every x-value. However, x-values can not be paired with multiple y values. That is the tool we utilize when using the vertical line test. Can you draw a vertical line through any/every place on the graph and not have it cross more than once?

In any case, your equation here can be rewritten as a function y(x):

y=12-x
y(x)=12-x

It is clearly a function.