If 5x + 15 ≥ 0, then |5x + 15| = 5x + 15. The equation reduces to
5x + 15 = 10x
15 = 5x
x = 3
If instead 5x + 15 < 0, then |5x + 15| = -(5x + 15), and the equation becomes
-5x - 15 = 10x
-15 = 15x
x = -1
If x = 3, then |5x + 15| = 30 and 10x = 30, so x = 3 is a valid solution.
If x = -1, then |5x + 15| = 10 while 10x = -10, so x = -1 is the extraneous solution.