Well, if you look at it like a "T," then by symmetry you know it has to be along the center line of the "T." In short, you take the moments of both rods and find the y-value where the sum of the moments is equal to zero.
In long, if the origin of the graph is at the base of the "T," follow these four steps:
1) Find the center of mass of each rod.
2) Find the total mass of each rod (which may be given)
3) Sum the moments of the rods. This means to multiply the mass of each rod by the distance each rod's center of mass is from a specified point. Specifically, to find the moment of one rod along the y-axis, multiply the mass by the y-value of a point, minus the y-value of the rod's center of mass. Do this to both rods, and sum the results. In a formula, it would look like this:
Total Moment = A(y - c) + B(y - d)
where:
A = Mass of rod 1
B = Mass of rod 2
c = y-value of the center of mass of rod 1
d = y-value of the center of mass of rod 2
4) Find the y-value where Total Moment equals zero. This will give you the center of mass of the "T" shape.
Judging by the additional details, it looks like your y-values are in terms of length. I'm too lazy to reverse-engineer your question from the additional details, but my method should still work using the "L" variable in the y-value.