First, we've got to make this into an equation with numbers, not words. We'll use x as "the number" when we make the equation. By "no more than," I'm assuming you mean to use the "less than or equal to" sign.
-4x + 9 ≤ x - 21
Now that we have the equation, we just have to solve for x.
-4x + 9 ≤ x - 21
-4x ≤ x - 21 - 9
-4x ≤ x - 30
-4x - x ≤ -30
-5x ≤ -30
-x ≤ - 6
x ≥ 6
In the end, x is greater than or equal to 6, because we had to change the entire equation from negative to positive.
