[tex]15. 7x^{4} - 5x^{4} = 2x^{4} \\17. 6b + 7b - 10 = 13b - 10\\19. y + 4 + 3 ( y + 2 ) = y + 4 + 3y + 6 = 4y + 10\\21. 3y^{2} + 3 ( 4y^{2} - 2 ) = 3y^{2} + 12y^{2} - 6 = 15y^{2} - 6\\23. 0.5 ( x^{4} - 3 ) + 12 = 0.5x^{4} - 1.5 + 12 = 0.5x^{4} + 10.5[/tex]
