F'(x)=12x^3+15x^2
12x^3+15x^2=0
4x^3+5x^2=0
x^2(4x+5)=0
x1=0, x2= -5/4 -- критические точки
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↓ -5/4 ↑ 0 ↑
xmin= -5/4
y(-5/4)=3*(-5/4)^4+5*(-5/4)^3+1= -1.44140625=ymin
y(-2)=3*16-5*8+1=48-40+1=9
y(2)=3*16+5*8+1=48+41=89=ymax