[tex]\left(3+\sqrt{-16} \right) \left(5-\sqrt{-25} \right)\\\\=\left(3+\sqrt{-1} \cdot \sqrt{16} \right) \left(5- \sqrt{-1} \cdot \sqrt{25} \right)\\\\=(3+4i)(5-5i)~~~~~~~~~~~~~~;\left[i=\sqrt{-1} \right]\\\\=15-15i + 20i - 20i^2\\\\=15+5i -20(-1)~~~~~~~~~~~~;[i^2 =-1]\\\\=15+5i +20\\\\=35+5i[/tex]
