Express the given complex number in the form $a+i b:(5 i)\left(-\frac{3}{5} i\right)$
$(5 i)\left(\frac{-3}{5} i\right)=-5 \times \frac{3}{5} \times i \times i$
$=-3 i^{2}$
$=-3(-1) \quad\left[i^{2}=-1\right]$
$=3$
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