The correct response is: The expressions are equivalent because Ella got the same result when she substituted zero for f.
This conclusion can be drawn from Ella's calculations. When she substituted \( f = 0 \) into both expressions, she obtained the same result of \( 3.6 \). Since this is true for at least one value of \( f \), and we don't have evidence of them being different for all values, the expressions can be considered equivalent based on this specific point.