Answer:
f(3) = (-∛150)/10
Explanation:
Put 3 where x is and evaluate. If you don't want the decimal, you can rationalize the denominator to get an exact form.
f(3) = ∛(3/(-7·3+1)) = ∛(-3/20)
= -∛(3·50/(20·50)) = (-∛150)/10 ≈ -0.5313292845913...
_____
Multiplying numerator and denominator by 50 makes the denominator 1000, a perfect cube.