Remember: PEDMAS
= -15x -10 -x+3 = -16x-20 + 13
= -16x -7 = -16x -7
well there it is....both sides are equal !
Simplifying by distributing the 5 the understood -1 and the -4
-15x -10 - x + 3 = -16x -20 + 13 combine like terms
-16x - 7 = -16x - 7
Note that any x makes this true.....so....the "solution" is all real numbers