Explanation:
we find the slope as factor of x in an equation like this
y = ax + b
we have here 6x + 4y = 64
we need to bring this into the standard form above :
4y = -6x + 64
y = -6/4 x + 16 = -3/2 x + 16
so, we know the original slope is -3/2.
remember, the slope is the ratio "y coordinate change / x coordinate change".
anyway, the perpendicular slope is turning the original slope upside-down and flips the sign.
so, it is
2/3