What is 8 - ab an example of?

What is c - 4 > 18 an example of?

What is 3d = 18 an example of?

What is d - 5 = 2d an example of?

 $a\ =\ \pi r^2$ is an example of?

 $s\ =\ \frac{d}{t}$ is an example of?

 $6x\ \equiv\ 4x\ +\ 2x$ is an example of?

 $2\left(x+4\right)\ \equiv\ 2x\ +\ 8$ is an example of?