What is in numbers
doubling the quantity of a number decreased by six is less than negative thirty-six.

Let us assume the unknown number to be = x
Then we can easily write the equation as
2x  - 6 = -36
Dividing both sides of the equation by 2 we get
x - 3 = - 18
x = - 18 + 3
x = - 15
So the unknown number that we get is -15. I hope the procedure is clear enough for you to understand. You can always use this procedure for solving similar question in future without taking any outside help.