u-v/8v - 6u-3v/8v

There are no bracket so this is your question

u-v/8v - 6u-3v/8v

$$\\u-(v/8)*v - 6u-(3v/8)*v\\\\ =u-\frac{v^2}{8}-6u-\frac{3v^2}{8}\\\\ =u-6u-\frac{v^2}{8}-\frac{3v^2}{8}\\\\ =-5u-\frac{4v^2}{8}\\\\ =-5u-\frac{v^2}{2}\\\\$$