App for plotting a function with unknown constants?

Hi

For Physics, I had to differentiate a function and equal it to zero to find extreme cases.

I was supposed to find 2 answers a minimum...x=0 and a maximum... x=d/(2^0.5)

After solving the equation...I only found the max.

I know x=0 has to be a solution as well, because that solution is quite logical in this scenario..I was calculating magnitude of Electric Field Strength as a function of x..and at x=0..E has to be 0 in this case.

So finally my question:

I want to plot the function I found, but I dont know a website or app that can let me do that.

So I can see only the shape of the function.

For example...see a parabola when I put in ax2 +bx+c

Is there an app for that

Thanks

PS Sorry if I wrote bad English...is not my main language.