(7x^3/2) - (4x^3/2)

If it were 7x - 4x, you'd have 3x. Similarly, 7x^3/2 - 4x^3/2 = 3x^3/2.

May be math.not.maths's interpretation is correct, but I have another:

$\dfrac{7x^3}{2}-\dfrac{4x^3}{2}\\ =\dfrac{3x^3}{2}$