An equation is a mathematical statement containing an equals sign.

An equation declares that something is equal to something else.

An equation can be solved.

Here is an example of an equation:

$$y = \frac{4 × 5}{10}$$

An expression is not an equation, because expressions do not contain equals signs.

Here is an example of an expression:

$$33y + 2$$

A formula is not an equation because a formula cannot be solved.

Here are some formulas:

$$speed = \frac{distance}{time}$$

$$A = \frac{b × h}{2}$$

$$C = \pi D$$

Of course, when we use formulas, we substitute numbers into them.

Once that happens, they stop being formulas and turn into equations, which can be solved.