Find the equation of the line with gradient $3$ passing through the point $(1, 2)$.

Using $y - y_1 = m(x - x_1)$: $y - 2 = 3(x - 1)$, so $y = 3x - 3 + 2 = 3x - 1$. Writing $y = 3x + 2$ treats the $y$-coordinate of the point as the intercept, which is only true when the point is on the $y$-axis.