unfortunately, you can't, since as you've noticed, you don't get a specific unit as an answer. Theoretically, you could write 900(mm*cm), but the correct approach is to convert both measures to the same unit.
It's like if you want to know how many feet you walked in one hour if you know you walk 3 miles an hour. You must convert one or the other before finding an answer
using this scale :
km hm dam m dm cm mm
<-/10 *10->
each conversion you do going left, you divide by 10. each you do going right, you multiply by ten. You can check on your ruler, you'll see that there are 10 mm in 1 cm.
However, this is only good if these measures are NOT squared. if they are, you change the 10 by 100 (and if it's cubic, then it's 1000 instead of 10)