If rs=8y+4 , st=4y+8 , and rt=15y-9, find rs, st, and rt
someone me

First write your equation to solve for y. You know RS plus ST equals RT. 
8y + 4 + 4y + 8 = 15y - 9

Next combine like terms on the left side of the equal sign. 
8y + 4y + 4 + 8 = 15y - 9
 
Then add the like terms.
12y + 12 = 15y - 9

Simplify using algebra. Subtract 12y from both sides. 
12y + 12 = 15y - 9
-12y          -12y
12 = 3y - 9

Now add 9 to both sides.
12 = 3y - 9
+9          +9
21 = 3y

Divide both sides by 3. Now you have your y value.
21 = 3y
/3     /3
7 = y

Now that you have the y value (7), just plug it into the equations for each side. 

RS:
8y + 4
8 * 7 +4
56 + 4
60
RS = 60

ST:
4y + 8 
4 * 7 +8
28 + 8
29
ST = 36

RT:
15y - 9
15 * 7 - 9
105 - 9
96
RT = 96

We know these are correct because RS + ST should equal RT. Does 60 + 36 = 96? Yes. 

Hope this was of help!

Element 17 is chlorine it is a group 7 element(halogen) and belongs to period 3 of the periodic table