In Scientific Notebook, we use Compute → Solve → Exact:

x4 + 0.4x3 − 6.49x2 + 7.244x − 2.112 = 0 ,

Solution is: {`x = -3.2`}, {`x = 1.2`}, {`x = 0.5`}, {`x = 1.1`}

Here is the graph:

123-1-2-3-4510-5-10-15-20-25-30xyOpen image in a new page

Graph of y = x4 + 0.4x3 − 6.49x2 + 7.244x − 2.112

The three positive roots are difficult to see. Here is that portion again, zoomed in for a clearer view:

0.20.40.60.811.21.40.050.1-0.05-0.1xyOpen image in a new page

Graph of y = x4 + 0.4x3 − 6.49x2 + 7.244x − 2.112