make this into an equation

There are a couple of things to notice here. You don’t have a starting temperature, so that will be an unknown. I will call it S.

The next thing to notice is that the current temperature (C) at any given time is just the starting temperature minus 4 times the number of hours (H) that have passed since 1 PM.

C = S – 4*H

We want to solve this for H when the current temperature (C) is -1.

-1 = S – 4*H

You can rearrange this by adding 4*H to both sides and adding 1 to both sides:

4*H = S + 1

Then you divide both sides by 4

H = (S+1)/4

This will tell you the number of hours PAST 1 PM that it takes to get to -1 degrees. Of course, you need to know the starting temperature, S, in order to get a numeric answer.

Let’s check this with an example. If the starting temperature were exactly 4 degrees above -1, then it should take exactly 1 hour. So let S = -1 + 4 = 3. If you plug 3 in for S, you do see that H becomes 1 hour PAST 1 PM (2 PM).