Firt, why are you writing rootfinding code, when tools like fzero exist? Don't write poorly written code when well written code already exists, and you already have that code!
Next, don't write things like a solver inline. Learn to use functions. Learn how to write your own function m-files, that contain code, something you will use more than once.
Next, whatever solver you use, you shpuld understand the solution you find depends on the starting value. Here you found a solution when you started at x=-1.8. Had you started the solver at a different, intelligently chosen point, you would have found the other solution. So the answer is rather simple, just call the solver TWICE, with different start points.
But don't use Newton's method. Don't write your own code for something that already exists. That means you need to start learning what tools are available already in MATLAB. You paid for the package, so why not learn to use the full capability of that tool? fzero is is not even part of a toolbox, but in MATLAB itself.