When writing twenty-one-and-a-half-trillion in scientific notation, what is the exponent needed on the base ten?I also need an explanation on how to find this!

1 trillion is 10^9.

Thus, 21 1/2 trillion is    21 1/2 times 10^9, or 2.15 times 10^10

Hope this helps.  If not, ask for clarification.