We can begin solving this equation by isolating x on one side of the equation. We do this by adding 10 to both sides and subtracting 6x from both sides:
8x - 4 = 6x - 10
8x - 6x = -10 + 4
2x = -6
x = -3
Now that we have found the value of x, we can substitute it into the expression 5x to find its value:
5x = 5(-3) = -15
Therefore, the value of 5x is -15. So, the correct option is (b) -15.