Question 24681
Your teacher must have explained to you that i (for imaginary) is the symbol used to represent{{{sqrt(-1)}}}, so:

{{{x^2 - 16}}} Take the square root of both sides.
x = + or - {{{sqrt(-16)}}}  Simplify.
x = +or-{{{sqrt(-1*16)}}}
x = +or-{{{4sqrt(-1)}}}
x = +or-4i

The other problems are handled in a similar fashion.