# Expansions!

Discrete Mathematics Level pending

The expression$(2a^2 + 3b)^9 - k(3a + 2b)^{12}$where $$k$$ is a real number, has 21 terms in its expansion. If the value of $$k$$ (in simplest form) is of the form $$\frac{a}{b}$$, where $$a$$ and $$b$$ are positive integers, find the value of $$a+b$$.

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