While you do that why not add more features, like roots of a function regardless of its order or something else. You can use the MATLAB function roots which will help immensly.
Currently, even if you fix these mistakes, it's still very bare bones and needs more features.
I gave two stars because by running this program, the root is changed then not changed back. That is not a good way to make users of your code happy. When the user closes the GUI, you need to set colordef back to what it was prior to initialization. I will change my rating when this is fixed.
In the lines where you calculate the quadratic formula, you need a parenthesis around the 2*a, like this;
x1=(-b+sqrt(b^2-4*a*c))/(2*a)
x2=(-b-sqrt(b^2-4*a*c))/(2*a)
Otherwise your solutions are only valid for a = 1.