Hey guys, I just need some quick help! Can anyone explain to me how to find the square root of this number... or pretty much any other number in general?

Like, I know for a fact that this is 4. But my teacher stated that I cannot find the square root of a negative number. This, for some reason, turns to a POSITIVE 4 and I just do not understand why. Can you guys, like, you know, explain?

Ariana starts with 100 milligrams of a radioactive substance. the amount of the substance decreases by 20% each week for a number of weeks, w. the expression 100(1−0.2)w finds the amount of radioactive substance remaining after w weeks. which statement about this expression is true? a) it is the difference between the initial amount and the percent decrease. b) it is the difference between the initial amount and the decay factor after w weeks. c) it is the initial amount raised to the decay factor after w weeks. d) it is the product of the initial amount and the decay factor after w weeks.