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G(-3+h) tells you to substitute "-3+h" for "t" in G(t):
G(-3+h) = 5-3(-3+h) = 5+9-3h = 14-3h
Next find G(-3) by substituting -3 for "t" in G(t)
Take that result and subtract it from G(-3+h) (= 14-3h)
I got an answer of -3h