a) $\mathbf{a}-j 5(3+j 4)$
We can distribute the $-j5$ to both terms inside the parentheses:
$\mathbf{a} - j5(3) - j5(j4)$
Now, we know that $j^2 = -1$, so we can simplify:
$\mathbf{a} - 15j - 20$
So, the expression in the form $a + jb$ is:
$\boxed{\mathbf{a}
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