[(3x+4y)^2-3x(3x+4y)]÷(-6y)
=(3x+4y)(3x+4y-3x)÷(-6y)
=4y(3x+4y)÷(-6y)
=-2(3x+4y)/3
=-2x-8y/3
原式=-2×(-1)-8×3/3
=2-8
=-6