What is the value of a, if a^2 – 64 = 0 and a > 0?
a) 0
b) 4
c) 8
d) 16

A^2-64=0 you can factor like this:

(a+8)(a-8)=0 and since a>0, a must be 8

or you can do this:

add 64 to both sides of the equation leaving you with:

a^2=64  taking the square root of both sides

a=(-8,+8) and since a>0, a=8

The answer to the question is d