The following equations are linear: $$\Tred{x}-1 = 0, \quad 3\Tred{ t } + 2 = -2, \quad \frac{\Tred{y}}{2} + \frac{4}{5} = -\frac{\sqrt{3}}{7}$$
The following equations are not linear: $$\Tred{x}^2 = 1,\quad \sqrt{\Tred{t}} - 1 = 0,\quad \frac{1}{\Tred{y}} + \frac{1}{\Tred{y}-1} = 1$$
The general form of a linear equation is $$\Tgreen{a} \Tred{x} + \Tgreen{b} = \Tgreen{c}$$ for numbers $\Tgreen{a}\ne0$, $\Tgreen{b}$ and $\Tgreen{c}$.