Six more than five times a number is at least twenty-one

Lets say the number is x so we can make an equation

If is at least 21, that means it has to be greater then or equal to it.

6+5*x>=21

You can subtract 6 from both sides

5*x>=15

Then divide both sides by 5

x>=3

You can conclude that the number is greater then or equal to 3.