Straight lines are easiest to compare when written in the form $y=mx+c$. In this form, $m$ represents the gradient and $c$ represents the y-intercept.

If the equation is not already in this form, you must make $y$ the subject first. For example, $6y=3x-12$ becomes $y=\frac{1}{2}x-2$, so the gradient is $\frac{1}{2}$.

Parallel lines have the same gradient, while perpendicular lines meet at a $90^{\circ }$ angle. In most coordinate geometry questions, your first goal is to determine the gradient correctly.