I would choose to have a higher initial rate of zombies with a slower growth rate because (although it would be difficult in the beginning) i would have more time to prepare for the zombies.
Firstly, expand the equation.

Next, move the variable x to the left and all the constant to the right.

Then, solve for x.
Check the picture below.
since the second figure is also a square, then the sides touching the diagonals have to be all equal, and that'd only happen if those sides bisects the larger square's diagonal.