f(-6) is the point where the line hits -6 on the x axis, which in this case is (-6,6). It's easier to think of a line as an equation; this line would be f(x) = |x|. Absolute value is the magnitude of a number without regard to its sign, so the absolute value of -6 is 6, because you're ignoring the negative sign. I hope this helps you with future problems!
