When we use algebra, we use letters to stand for numbers that we are trying to find.
Algebra is like a language with its own rules.
a stands for a number.
a + 2 is 2 more than the number.
a - 7 is 7 less than the number.
5a is 5 times the number. (N.B. We don't use the × sign in algebra because it looks like the letter x.)
a/4 or ¼a is a quarter of the number or the number divided by 4. (N.B. We don't use the ÷ sign in algebra.)
I think of a number.
I double it and take away 5.
I get 21.
Write this using algebra.
Use n for the number I think of.
n is the number I think of.
Double it to get 2n.
Take away 5 to get 2n - 5.
This equals 21.
So we write:
2n - 5 = 21