Hello there!
To solve a function by graphing would be to graph it and then look at where it intersects the x-axis. This way is not always accurate due to the fact that one cannot graph it to the exact point all the time, and even if they can or use a graphing calculator, it's difficult to read the graphs too. For those two reasons, solving by graphing throws off the answer most of the time, therefore this method is rarely reliable and it's important to learn other ways to solve.
I really hope this helps!
Best wishes :)