Here is question 7
Another method i have seen is to try and get unwanted units to cancel out. that is, we multiply by the conversion factors as fractions. if a unit we don't want is in the top, we multiply by a fraction where that unit is in the bottom, so it cancels, and vice versa.
so to do the last question by that method. we know 1 mm^2 = 10^-12 km^2
=> we can take 1 = (10^-12 km^2)/(1 mm^2) ...i divided both sides of the previous equation by 1 mm^2. that way, we can multiply by (10^-12 km^2)/(1 mm^2), which is the same as multiplying by 1, so we don't change anything. but then, since mm^2 is in the top of the first fraction, it will cancel with the mm^2 in the bottom of this fraction, and leave km^2 in the end