Plug-'n-Chug.
If f(x) = -3x+8
f(x+h) = -3(x+h)+8
Put it all together and simplify it.
Hmmm...Are we forgetting the Distributive Property of Multiplication over Addition?
[-3(x+h)+8] - [-3x+8] = [-3x - 3h + 8] - [-3x+8]
= -3x - 3h + 8 + 3x - 8
= -3x + 3x - 3h + 8 - 8
= (-3x + 3x) - 3h + (8 - 8)
Finish up and then ask again. I'm just not seeing 'h' alone in the numerator.