For this case, the first thing we must do is define variables.
We have then:
x: number of minutes
y: final temperature
We write then the equation that models the problem:

For y = 20 we have:

Clearing x:
Answer:
The temperature of the water will be 20 degree celcius after 150 minutes
Y = 3x + b
Now plug in points
-7 = 3(-1) + b
-7 = -3 + b
b = -4
The y intercept is -4