When you see COSB = 3/5 in a right triangle, you should think of the number of triangles: 3, 4, 5, b-a=3, then multiply B-A = 3 at the same time, that is, each side is 9 12, 15, so C =15; Let two of them be x 1, x2, then Vieta's theorem obtains: x 1+x2 = 3 (m+ 1), X 1x2 = 20, x 1? +x2？ = 15? = 225, then x 1? +x2？ =(x 1+x2)？ -2x 1x2=9(m+ 1)？ -40 = 225, so (m+ 1)? = 265/9, so m = √ 265/3- 1 or m =-√ 265/3- 1.

I hope it helps you!