Answer
(a) true
(b) false (if number is 0)
(c) true
Work Step by Step
(a) This is true, since $a=b$ is equivalent to $a+y=b+y$.
(b) This is true if the number is non-zero, since $a=b$ is the same as $a*x=b*x$. But if the number is 0, the equation will remain true, but will not be equivalent.
(c) This is false, since squaring removes negatives. For example $-1=1$ is false, but $(-1)^2=(1)^2$ is true.
