Well if its exponential it must be x ^n
Use Pythagoras's theorem.

Now, minus 12 from 15:
15 - 12 = 3,
which you need to square.
So the length of a = 9cm.
You can also do:

Which gets 9.
Hope this helps.
Answer:

It's change of subject. :)
Part A:square cross-section
<span>Part B: a Rectangle cross -section</span>
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Perpendicular angles are where 2 lines form 90 degree angles, such as in C