To find the value of 6x - 8y when x = 2/3 and y = 5/8, we need to substitute these values into the equation.

6(2/3) - 8(5/8)
= 12/3 - 40/8
= 4 - 5
= -1

Therefore, when x = 2/3 and y = 5/8, the value of 6x - 8y is -1.