a, $(5x+1)^{2}$ -(5x+3) =30
<=>$25x^{2}$ +10x +1 -($25x^{2}$-9) =30
<=> $25x^{2}$+10x+1-$25x^{2}$+9=30
<=> 10x+10=30
<=> 10x= 20
<=> x = 2
Vậy x=2
b, (x+3)(($x^{2}$-3x+9)-x(x-2)(x+2)=15
<=>$x^{3}$ +27-4x-$x^{3}$=15
<=>($x^{3}$-$x^{3}$)+4x+27=15
<=> 4x+27 =15
<=> 4x =-12
<=> x = -3
Vậy x =-3