$x \lt 4$ Please see image
Work Step by Step
$ 15-3x\gt 3\qquad$ ...add $-15$ to each side of the inequality. $ 15-15-3x\gt 3-15\qquad$ ...simplify. $-3x\gt-12\qquad$ ...divide with $-3$. ...dividing with a negative inverts the inequality sign. $x \lt 4$ The solutions of the given inequality are all real numbers smaller than $4$. Smaller numbers are to the left of $4$. Use an open dot in the graph to indicate $4$ is not a solution.