$$(5-2i)(3+3i)=a+bi$$

In the equation above, $a$ and $b$ are real numbers and $i=\sqrt{-1}$. What is the value of $a$?

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$$(5-2i)(3+3i)$$ $$=5(3)+5(3i)-2i(3)-2i(3i)$$ $$=15+15i-6i-6i^2$$ $$=15+9i-6(-1)$$ $$=15+9i+6$$ $$=21+9i$$ $$\colorbox{aqua}{$21$}+9i=\colorbox{aqua}{$a$}+bi$$