Answer
$a^2 +2ah-3a+h^2-3h$
Work Step by Step
If $f(x) = x^2-3x$
$f(a+h) = (a+h)^2-3(a+h) = a^2+2ah+h^2-3a-3h = a^2 +2ah-3a+h^2-3h$
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