How do I resolve this stability criterion question?

I really don't understand how to do this following steps: Ask the user to choose a time step, the code should prevent the stability criterion to be broken. As the user to choose the grid spacing

I only got apha= 6,424079066 * 10^-5 but I don't have At and neither I have (Ax)^2 which I know it is the grid spacing h, so I really don't know how to resolve this.

Answer 1

[Rafael Hernandez-Walls]