Note that RT is the whole line segment, and RS and ST are parts of it
RS = 6y + 5
ST = 2y - 3
RT = 12y - 14
Set the equation
RS + ST = RT
(6y + 5) + (2y - 3) = 12y - 14
Simplify. Combine like terms
6y + 2y + 5 - 3 = 12y - 14
(6y + 2y) + (5 - 3) = 12y - 14
8y + 2 = 12y - 14
Note the equal sign. What you do to one side, you do to the other. Isolate the variable (y). Subtract 12y from both sides, and 2 from both sides.
8y (-12y) + 2 (-2) = 12y (-12y) - 14 (-2)
8y - 12y = -14 - 2
Simplify. Combine like terms
(8y - 12y) = (-14 - 2)
-4y = -16
Isolate the y. Divide -4 from both sides
-4y/-4 = -16/-4
y = (-16)/-4
y = 4
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4 is your answer for y.
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hope this helps