**Equation**

An *equation* is a math sentence that says that 2 things are equal.
An equation always has an equal (=) sign. The thing or things that are on the
left side of the equal sign are equal to the things on the right side of the
equal sign. Here are a few equations:
X = Y + 5
A = B - 4
Z = 3 A
C = D (circumference of a circle)
A = W H (area of a rectangle with width W and height H)
E = M C^{2} (Einstein's famous equation)

In the first equation, if Y is 2, then X must be 2 + 5 or 7 to make the equation true.
In the second equation, if B is 10, then A must be 6 to make the equation true.
In the third equation, the number next to the letter means multiply the value of the
variable represented by "A" by 3. So if A is 4, then Z must be 12 to make the third
equation true.

Now here's a problem for you:

In the first equation, if Y were 10, what would X be?