At midday the temperature in Moscow was 7°C. At midnight it was -3°C.
By how many degrees did the temperature fall?

10 degrees C

Step-by-step explanation:

the difference between both temperatures is expressed by a subtraction

7 - -3 = 7 + 3 = 10

you could also do the subtraction the other way around :

-3 - 7 = -10

but the negative sign has no meaning for a distance between two points (a distance itself has no direction).

so, we are always using the absolute value of such difference calculations (when it is about lengths, distances, and so on, where the direction had no meaning).

and so, also the -10 is actually simply 10 for our problem here.

the temperature fell by 10 degrees.

answer:   x = 3 + √11     and     x = 3 - √11

step-by-step explanation:

x² - 6x + 9 = 11

(x - 3)² = 11

x - 3 = ±√11

    x = 3 ± √11

x = 3 + √11     and     x = 3 - √11