One number is - 3 times the first number. The sum of the numbers is - 4. What is the second number?

Lets say that the first number is x and the second number is y.

from the question we know that:

y=-3x and x+y=-4

from there, you need to replace the y in the second equation with - 3x since we know that is what y equals so we can have an equation with only one variable.

x+-3x+-4

Then you solve for x.

-2x=-4

x=2

then you find y with the fact that y=-3x

y=-6