[MATH]\text {Definition: } c < 0 \implies |\ c \ | = - c;\ c \ge 0 \implies |\ c \ | = c. [/MATH]
Good so far?
[MATH]\text {Let } d \text { be any real number.}[/MATH]
[MATH]\therefore d < 0, d = 0, \text { or else } d > 0.[/MATH]
[MATH]\text {CASE I: } d < 0 \implies \\
|\ d \ | = - d > 0 \implies |\ - d \ | = - d \implies |\ d \ | = |\ - d \ |.[/MATH]
[MATH]\text {CASE II: } d = 0 \implies \\
|\ d \ | = 0 = - 0 \implies |\ - d \ | = |\ - 0 \ | = |\ 0 \ | = 0 \implies |\ d \ | = |\ - d \ |.[/MATH]
[MATH]\text {CASE III: } d > 0 \implies \\
|\ d \ | = d > 0 \implies - d < 0 \implies |\ - d \ | = - (-d) = d \implies |\ d \ | = |\ - d \ |.[/MATH][MATH]\therefore \text {in all cases, } |\ d \ | = |\ - d \ |.[/MATH]
Any questions?
Now apply that theorem to your problem.