-r + 18 = -39 ... -r +18-18 = -39 + -18 = -r + 0 = -57. my study gude says the answer is 57 can someone please help me understand how the answer could be 57. and not -57
Hi Leona,
It's very simple. Watch
$\displaystyle -r+18=-39$
First we subtract 18 from both sides giving us:
$\displaystyle -r+18-18=-39-18$
$\displaystyle -r=-57$
But, we're not done. We must find a solution for r, not -r. To find r, we multiply both sides of the equation by -1.
$\displaystyle (-1)(-r)=(-1)(-57)$
$\displaystyle \boxed{r=57}$
To check this we start with our original equation:
$\displaystyle -r+18=-39$
Substitute 57 in for r to get:
$\displaystyle -(57)+18=-39$
Notice, the negative sign was already part of the equation. I just replaced r with 57. Now the check is fine.
$\displaystyle -57+18=39$
$\displaystyle -39=-39$