-6x + 3 + x - 7...when ur adding expressions such as this, and there is a plus sign in between them, the parenthesis are not needed.
we now combine like terms...
-6x + x = -5x
3 - 7 = -4
and we are left with : -5x - 4 <==
It will take Marvin 12 days to exersise for 576
The formula for slope is 
In this case:

so...


-1 <<<The slope
Hope this helped!
~Just a girl in love with Shawn Mendes
Based on the picture, PC = PD