Đáp án: a) y=f(-5) = 3.(-5)²-7
= 68
y=f($\frac{1}{3}$) = 3.$\frac{1}{3}$² -7
= $\frac{-20}{3}$
b) y=5 ⇔ 5=3x²-7
⇔ 3x²=5+7
⇔ 3x²= 12
⇔ x² = 4
⇔ x = 2
y=6$\frac{2}{3}$= $\frac{20}{3}$ ⇔ $\frac{20}{3}$= 3x²-7
⇔ 3x² = $\frac{20}{3}$+7
⇔ 3x² = $\frac{41}{3}$
⇔ x² = $\frac{41}{3}$ .$\frac{1}{3}$
⇔ x² = 41/3
⇔ x = $\frac{√123}{3}$