Question 191914
equation:
x + (x+1) + (x+2) = y
use the equation to help you write the missing expressions in terms of y.
combine terms and solve for x
3x + 3 = y
3x = y - 3
x = ({{{y/3}}}-1)
Substitute for x, and combine like terms in the following:
:
(x+5) + (x+6) + (x+7) =
(({{{y/3}}}-1)+5) + (({{{y/3}}}-1)+6) + (({{{y/3}}}-1)+7) =
3({{{y/3}}}) - 1 + 5 - 1 + 6 -1 + 7 = y + 15
:
: 
2x + 2(x+1) + 2(x+2) =
2({{{y/3}}}-1) + 2(({{{y/3}}}-1)+1) + 2(({{{y/3}}}-1)+2  =
{{{2y/3}}}-2 + {{{2y/3}}}-2 + 2 + {{{2y/3}}}-2 + 4 = 
{{{6y/3}}} - 2 - 2 + 2 - 2 + 4 = 2y
:
:
(x+a) + (x+1+a) + (x+2+a) =
{{{y/3}}}-1+a + {{{y/3}}}-1+1+a + {{{y/3}}}-1+2+a = 
3({{{y/3}}}) - 1 - 1 + 1 - 1 + 2 + a + a + a = y + 3a
:
;
did you understand these steps?