Calculus Help: Determine whether the given set of functions is linearly independent on the interval

Calculus Help: Determine whether the given set of functions is linearly independent on the interval

Determine whether the given set of functions is linearly independent on the interval (-∞,∞)
f_1 (x)=x, f_2 (x)=x^2 , f_3 (x)=4x- 3x^2

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