You know the length of the entire line segment, RT, to be 36. Since you know this, you can take the line segments inside it (RS and ST) and make them add up to RT in an equation.
Your equation should look like: RS + ST = RT; plug the expressions for RS, ST, and RT into the equation. Now your equation should look like this:
(6x + 1) + (x + 7) = 36
Combine like terms (6x + x) and (1 + 7).
7x + 8 = 36
Subtract 8 from both sides.
7x = 28
Divide both sides by 7.
x = 4
Your answer to this problem is x = 4.