Let x-3 = y. Then your equation becomes:
A product of 2 factors equals zero if one factor equals zero:
That means: x-3 = 0 ==> x = 3 or x-3=2 ==> x = 5
You must plug in the result into the original equation to check if the result is a valid solution:
x = 3:
x = 5:
Therefore: Only x = 3 is a solution.