How do I write an extension method for a generic type with constraints on type parameters?

I'm working with a task specific .NET plattform, which is precompiled and not OpenSource. For some tasks I need to extend this class, but not by inheriting from it. I simply want to add a method.

At first I want to show you a dummycode existing class:

public class Matrix<T> where T : new() {
    ...
    public T values[,];
    ...
}

I want to extend this class in the following way:

public static class MatrixExtension {
    public static T getCalcResult<T>(this Matrix<T> mat) {
        T result = 0;
        ...
        return result;
    }
}

I've got this syntax from many google links so no idea whether it is correct. The compiler tells me no error, but in the end it doesn't work. In the end I want to call this function in the following way:

Matrix<int> m = new Matrix<int>();
...
int aNumber = m.getCalcResult();

So anyone got an idea? Thank you for your help!

Regards Nem