(2a+c)(a-3c)+a(2c-a) = 2a^2 - 6ac + ac - 3c^2 + 2ac - a^2= a^2 - 6ac + ac -3c^2 + 2ac - a^2= -5ac - 3c^2
(2x-b)(3x+b)+(3b-x)(b-x) = 6x^2 + 2xb - 3bx - b^2 + 3b^2 - bx - 3bx + x^2= 6x^2 - b^2 + 3b^2 + x^2= 7x^2 + 2b^2
(2a+c)(a-3c)+a(2c-a) = 2a^2 - 6ac + ac - 3c^2 + 2ac - a^2
= a^2 - 6ac + ac -3c^2 + 2ac - a^2
= -5ac - 3c^2
(2x-b)(3x+b)+(3b-x)(b-x) = 6x^2 + 2xb - 3bx - b^2 + 3b^2 - bx - 3bx + x^2
= 6x^2 - b^2 + 3b^2 + x^2
= 7x^2 + 2b^2