5x= 21-2x what does x mean

5x= 21-2x what does x mean

In primary schoool it might have been written like this.

$$5\times
\fbox{\begin{minipage}{0.6cm}\hfill\vspace{0.6cm}\end{minipage}}
=21-2\times
\fbox{\begin{minipage}{0.6cm}\hfill\vspace{0.6cm}\end{minipage}}$$

EXACTLY the same number has to go in both boxes to make the statement true.

The box has been replaced with the the letter x.  The idea is to solve the number sentence to work out what x is.  You will learn all sorts of techniques to be able ot do this easily.

Let's make this a bit easier to understand and to solve.

We will pretend that the the boxes are two identical bags of lollies and you want to know how many lolies is in each bag.

5bags = 21lollies - 2bags 

We can add 2 bags to both sides then we will have 

7bags = 21 lollies

7*3=21

So there must be 3 lollies in each bag.  3 goes in the the box.  

So if 

5x=21-2x then 7x=21 and x=3

I hope that helps.  

Think of x as a label on a box.  Inside the box is a number which you have to find by manipulating a balance (the equation).

On the right-hand side is a number equal to 21 with two boxes taken away.  Make this side 21 by adding two boxes to it.  To maintain the balance though, you also have to add 2 boxes to the left-hand side. So you get:

7x = 21

Now you have 7 boxes on the left-hand side, but you only want one, so divide the left-hand side by 7 to be left with just x. Again to keep the balance you also have to divide the right-hand side by 7, and since 21/7 is 3 you are left with:

x = 3.  This is the number in the box!