[tex] a(x+3) < 5x+15-x\\\\ax+3a < 5x+15-x\\\\ax+3a < 4x+15\ \ \ |-3a\\\\ax < 4x+15-3a\ \ \ \ |-4x\\\\ax-4x < 15-3a\\\\(a-4)x < 15-3a\\\\\text{The solution is all real numbers if}\ a-4=0\ \ \ |+4\\\\\boxed{a=4}\\\\\text{Check:}\\\\4(x+3) < 5x+15-x\\4x+12 < 4x+15\ \ \ \ |-4x\\12 < 15\ TRUE\to x\in\mathbb{R} [/tex]
