To find the cross product, you can set this up as a proportion. Please see the attached paper for one of the proportions you could use.
You would then multiply the numbers that are diagonal from each other.
You would have 21(8) = 12x or
168=12x.
To put it simply, irrational numbers are numbers that cannot be expressed as a fraction. In other words, irrational numbers are numbers that cannot be terminated(stop repeating) or continuously repeated as a single repeating number(.666 or .333 for example). So pi is a irrational number since it repeats but with different numbers repeating each time.
You could draw a parallelogram.
Trade association... and next time label this is in the correct subject