I got the inequality $-15\le3a+b\le-5$, with the condition that $-10\le a\le10$ and $-10\le b\le10$, and $a$ and $b$ are integers. Though I can solve it by bashing, I came across a video where the guy said that he solved it using lattice points on a graph, by substituting $x$ for $a$, and $y$ for $b$. I was interested to know how does this work.
Could anyone help me out with this? Thanks in advance.