21) 30⋅(-0,1)³+7⋅(-0,1)²-3,9

$$\begin{aligned} 30 \cdot (-0.1)^3 + 7 \cdot (-0.1)^2 - 3.9 &= 30 \cdot (-0.001) + 7 \cdot (0.01) - 3.9 \\ &= -0.03 + 0.07 - 3.9 \\ &= 0.04 - 3.9 \\ &= -3.86 \end{aligned}$$
Ответ: -3.86