C++ Help

[Gutholz]

It prints x1=NAN (not a number) because y1 is negative and sqrt() function can not calculate squareroot of negative numbers.

In first post you wrote:

Quote:ax4 +bx2+c=0

I assume you meant:
a(x^4) +b(x^2)+c=0
?

With the input from your screenshot:
a=1
b=4
c=1
The graph looks like this:

=> This function has no solution.
Try entering some other values, plot the graph, and compare, might be that your program is already correct?