a. $\frac{1}{2}x^{2} . \frac{6}{5}x^{3}$
= $\frac{1}{2} . \frac{6}{5} . x^{2+3}$
= $\frac{3}{5}x^{5}$
b. $y^{2}(\frac{5}{7}-2y^{2}+0.25)$
= $\frac{5}{7}y^{2+3} - 2y^{2+2} + 0,25y$
= $y^{2}(\frac{5}{7}-2y^{2}+0.25)$
c. $(2x^{2}+x+4)(x^{2}-x-1)$
= $2x^{4} - 2x^{3} - 2x^{2} + x^{3} - x^{2} - x + 4x^{2} - 4x - 4$
= $(2x^{2}+x+4)(x^{2}-x-1)$
d. $(3x-4)(2x+1)-(x-2)(6x+3)$
= $6x^{2} - 8x + 3x - 4 - 6x^{2} + 12x - 3x + 6$
= $(3x-4)(2x+1)-(x-2)(6x+3)$