Sheldon was doing a science experiment about temperature.he frist measured the temperature of some water and found it was 17 c.then he put thw water in the freezer and recorded the temperature two hours later.it had fallen to -11 c.what was the change in temperature in two hours?
if you guys answer me can you guys show example ? you

Sure. 

In order to go from the 17th floor of the building down to
11 levels underground, you have to go down 28 levels.

In order to go from 17 steps above zero down to 11 steps
below zero, you have to go down 28 steps.

Do you need another example ?

The temperature on August 1 was 90 degrees. 
The temperature on February 1 was -15 degrees. 
How far did the temperature swing altogether in 6 months ?

To go from 90 steps above ground down to 15 steps underground,
you have to walk down  (90 + 15) = 105 steps .

X/7 = 18/21
Lets Reduce:
21/7=3 so we know for every 3 in 18/21 we have one in x/7
This means 6/7=18/21
Check:
18/6=3
21/7=3