We want resolve 21*x^4 + 73*x^3 - 57*x^2 + 73*x - 78 = 0 by using equp.htm. We prove with a = -10 and b = 10, and click "test".

For x → ∞ and for x → −∞ a polynomial function of degree 4 tends to ∞.

We can draw the graphs with JavaScript; for example, see the script** x4321**
(see the code; se also here).

I can also use R. I would get:

or with PHP (see "a zoom" here):

or with Desmos:

But the outputs printed by the program (obtained by clicking "test") allow you not to draw the graph. Between −6 and 0:

until I get:

Similarly, between 0 and 2:

The solutions are −(4+1/3) and 0.857142857142857142... = 6/7 (I can use
this program to find the fraction equal to 0.857142857142857...:
M=857142, N=99999, ... -> 6/7).