When parabola and x axis have a common point, that is, y=0, find x.

That is 3 (x+ 1) 2-4 (x+ 1) = 0,

Calculate x=- 1, x= 1/3,

Then the coordinates common to the X axis are (-1 0) and (1/3,0). According to the equation, a parabola can be drawn directly.

2. Same as 1 method, y = 3x2+2x+c = 3 (x+1/3) 2+c-1/3.

According to the equation, we can see that the symmetry axis of parabola is x=-1/3, and the opening is upward.

So as long as x=- 1, y; =0 can guarantee that when-1

The formula is as follows:

3-2+c & lt； 0 ; 3+2+c & gt； =0,

These two formulas calculate -5.

Have you figured it out?