[tex]S[/tex] would be a basis for [tex]\mathbb R^3[/tex] if
(1) the vectors in [tex]S[/tex] are independent, and
(2) the vectors span [tex]\mathbb R^3[/tex].
[tex]c_1(5,4,-2)+c_2(-15,-12,6)+c_3(10,8,-4)=(0,0,0)[/tex]
These vectors are not linearly independent because if [tex]c_1=3[/tex], [tex]c_2=1[/tex], and [tex]c_3=0[/tex], we have
[tex]3(5,4,-2)+(-15,-12,6)=(15-15,12-12,-6+6)=(0,0,0)[/tex]
so [tex]S[/tex] is not a basis for [tex]\mathbb R^3[/tex].