# Question #cb305

In an equation the left side has to be equal to the right side, so you can choose a value of $x$ and evaluate the correspondent value of $y$ but the two sides must remain equal. Now if you choose $x = 0$ in your equation $y$ has to be equal to zero.
For example, consider the equation $y = 3 x$ this equation has $\infty$ solutions depending on the value of $x$ you choose.
Try to substitute $x = 0$, you'll get:
$y = 3 \cdot 0 = 0$
So for $x = 0 \to y = 0$
Graphically, a linear equation is represented by a straight line, and in the case of $y = 3 x$ is a straight line that passes through the origin (which has coordinates $x = 0$, $y = 0$).
Try by yourself using $y = 3 x$, choose various values for $x$ (say $- 2 , - 1 , 0 , 1 , 2$) and plot the values obtained as points of coordinates $x \mathmr{and} y$ on a Cartesian coordinate system.