What does this mean and what is it used for?

a+b=c
a+e=f
c=f

also tell me if this does not make sense

Okay, I'll set up a model equation and sow you how it works.

5 + 3 = 8 --- a + b = c
5 + e = f --- a + e = f

So we know c is 8.

Since c = f then f must be 8 also.

Now we just plug in 8 for f on the second equation which would look like

5 + e = 8

We know 5 + 3 is 8

So, therefore our answer is 3

And that's how you solve these equation formats. Hope this helped. Have a great day!