So the question is asking to solve for y
You know RS=7y-4
You know ST=y+5
You know RT=28
RS+ST=RT as shown in the diagram, This means that you can use that equation and replace RS,ST, and RT with what you know they are equal too. So you end up with the equation:
(7y-4)+(y+5)=28, Now you have to solve for y
7y-4+y+5=28
8y+1=28
8y=27
y=3.375