Help with imaginary numbers please!!!

Problem 1
------------------
Work Shown:
[tex]-\sqrt{49}+\sqrt{-64}\\\\-\sqrt{7^2}+\sqrt{-1*8^2}\\\\-\sqrt{7^2}+\sqrt{-1}*\sqrt{8^2}\\\\-7+i*8\\\\-7+8i\\\\[/tex]
=============================================
Problem 2
------------------
Work Shown:
[tex]\sqrt{-100}-\sqrt{24}\\\\\sqrt{-1*10^2}-\sqrt{4*6}\\\\\sqrt{-1*10^2}-\sqrt{2^2*6}\\\\\sqrt{-1}*\sqrt{10^2}-\sqrt{2^2}*\sqrt{6}\\\\i*10-2*\sqrt{6}\\\\-2\sqrt{6}+10i\\\\[/tex]
=============================================
Problem 3
------------------
Work Shown:
[tex]-\sqrt{-81}+\sqrt{16}\\\\-\sqrt{-1*9^2}+\sqrt{4^2}\\\\-\sqrt{-1}*\sqrt{9^2}+\sqrt{4^2}\\\\-i*9+4\\\\4-9i\\\\[/tex]
=============================================
Problem 4
------------------
Work Shown:
[tex]\sqrt{-1}*2\sqrt{16}\\\\\sqrt{-1}*2\sqrt{4^2}\\\\i*2*4\\\\8i\\\\[/tex]
We could express this as 0+8i, but I think 8i looks much cleaner.