Geometry
posted by Tracy .
Angle JKL is congruent to angle MNP, KL= 21x  2, NP = 20x, LJ = 15x, PM = 13x + 4. Find LJ.
I don't understand how to do it! Please, help! :)

Because the two triangles are congruent, the following pairs of side lengths are equal: JK = MN, KL = NP and LJ = PM
The last two relationships tell you that
21x  2 = 20x, and 15x = 13x + 4
Both of those equations tell you that x = 2. (The gave you more information than you need to solve the problem!)
Since x = 2, LJ = 4 
m angles mnp=23 degrees