Answer:
x = 1 / (1/3 - R)
Explanation:
Let's call Robert R, and Juan J.
6R = 1 (think of 1 as 100% of the job done). call 6R = 1 equation '1'.
xJ = 1, J = 1/x (call this equation '2').
if Robert and Juan can do it together in 3 hours, then:
3(R + J) = 1
3R + 3J = 1
3J = 1 - 3R
J = 1/3 - R
1/x = 1/3 - R
x = 1 / (1/3 - R)