There are two possible solutions because a power is involved. This can be shown in the graph. When we graph to the power of 2, we get a parabola. This gives us two x-intercepts, giving us two answers.
And why is this? When you square a negative number, you get a positive number since a negative times a negative is a positive.
Example: 2 * 2 equals 4, and -2 * -2 also equals 4. This means both 2² and (-2)² are equal to 4.
This means that we will get a positive and negative number value that both equal the same thing when being squared, which is why you get two possible solutions to this equation.