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.

Here for a good time of free question aha