(3+5x)^2 - 6(5x+2) = (3+5x)(3+5x) - 6(5x+2)
= (3^2 + 235x + (5x)^2) - (30x + 12)
= (9 + 30x + 25x^2) - 30x - 12
= 9 + 25x^2.