So the parent function is y=x, right? y=-2x+6 is different because the slope is -2 (which flips the line so it's now going from left to right while also being more "taller", and the line's y-intercept is 6, while on the parent function, the y-intercept is 0.
Parent function: y = x (or y=mx+b)
Your function: y = -2x+6
6 = y-intercept (b)
-2 = slope (m)
Hope this helps!