The variable x has had the following done to it:
• multiply by -1/5
• add 9
We want to "undo" these operations in reverse order.
• undo "add 9" by "add opposite of 9"
.. -x/5 = -6 . . . . . . . -9 added to both sides of the equation
• undo "multiply by -1/5" by "multiply by -5". We choose -5 because it is the multiplicative inverse of -1/5. That is (-1/5)*(-5) = 1.
.. 1x = (-5)*(-6)
.. x = 30