factor (3x+1)^2-(x-4)^2
Let's factor in ℝ: (3x + 1)2 -(x -4)2
We recognize a difference of two squares: (3x + 1)2 -(x -4)2. Apply a2 -b2 = (a -b)·(a + b), with a = 3x + 1 and b = x -4
You can factorize this expression into: ((3x + 1) -(x -4))·((3x + 1) + (x -4))
ℹ-(x -4) = -x + 4‖1 -4 = -3‖3x + x = 4x
(3x + 1 -x + 4)·(4x -3)
ℹ1 + 4 = 5‖3x -x = 2x
▶Factored expression:   (2x + 5)·(4x -3)