It's a rational number. 9 is a perfect square. What you have asked is written as -sqrt(9), which can be written as -sqrt(3*3) which is - 3. That's perfectly OK. What would not be OK is something like -sqrt(10) which is irrational because it cannot be represented as a fraction or a whole number or a repeating decimal or a decimal that terminates.
She would've put 11 dimes in each pile. 55/5=11
units like radius, height, width, length or segments are single units, like meter or feet.
areas are double units, so they'd be in say meter² or feet².
volumes are triple units, namely like meter³ or feet³.
