As a recent student, I always needed help......
Basic truss solving methodology: Find external reactions first, then dive into truss to find internal forces if needed for a problem.
For this problem you can start by taking a moment down at L5. Taking a moment at this point leaves only 1 unknown item, which is the x-component of the force at U4.
The vertical component of the reaction at U4 goes through L5 so it creates no moment. All that is unknown is the horizontal component at U4, which you can solve for.
Once you have the horizontal component at U4, you can find the axial force in element U3-U4 by using sines/cosines. Or you can use sines/cosines to find the vertical reaction at U4. You know the direction of the force at that point, which is along the line U3-U4 and you also now know the x-component of this force.
Hope that helps.....