why does the sign change from greater than or equal to to less than or equal to
Answer provided by our tutors
If each side of an inequality is multiplied by the same negative number (in your case it is '-1'), then the sign will flip, or in your case from greater than or equal to less than or equal to.
This can be also explained another way: if we add 'a + 9' on both sides of the inequality we get
Or this is the same as