So, because mJDK and mJDA lie on a line, they must be supplementary, and since mJDK is 90 degrees, mJDA must be too. So, you know that the combination of mADC and mCDJ must be 90 degrees. 4x+3x-8=90 simplified is 7x=98, which says that x is equal to 14. So you plug in the values for 3x-8, and voila! mCDJ is 34!