(4 /10x) - (2x + 8)/5 = 4/5
Multiply by 10x.
4 - 2x(2x + 8) = 2x(4)
4 - 4x^2 - 16x = 8x
4 - 4x^2 - 16x - 8x = 0
-4x^2 - 24x + 4 = 0
-4(x^2 + 6x - 1) = 0
x = [-6 +/- sqrt(6^2 - 4*1*-1)]/2
x = [-6 +/- sqrt(36 + 4)/2
x = [-6 +/- sqrt 40)/2
x = [-6 +/- 2 sqrt 10]/2
x = -3 +/- sqrt 10