Every mathematical sentence with an equal sign is an equation, for example:

• For a rectangle with length l, width $b$ and a perimeter of $100$ it follows that: 2l + 2b = 100.

• For a square with an area $A$ and side $z$ it follows that: A = z ².

• For the number of copies $a$ per month, it follows that 250 + 0.06 · a = 0.10 · a.

• A number and its square add up to 90, what is that number ?

The first two equations describe a relation between two variables. Here you often use graphs.

In the latter two examples you can solve the equation.
That means finding a value for the variable that will make the stated sentence, the given equation, true.
There are different techniques, like the balance method and the inverse calculation method. It is also possible to solve equations with your graphic calculator.