(must show working) if a square has an area of 81cm² then what are the lengths of its sides in metres centimetres and millimetres?

First of all, keep in mind :
m --> cm = times by 100
cm --> mm = times by 10
mm --> cm = divide by 10
cm --> m = divide by 100

so we know that a square has 4 equal sides and to find the area of a square we do base*height. now these two numbers on the base and height are the same so we're basically squaring them to get the area of 81. therefore, we do the reverse so square root 81 and we get the answer 9cm

now let's convert 9cm to mm
= 9*10 = 90
area in mm = 90mm

now let's convert 9cm to meters
= 9/100=0.09
area in m = 0.09m

and finally, we already worked out the area in cm
= 9cm

hope this helped :)