a)
because Plane‖3x-y+z=10
so assume the Plane as 3x-y+z=10+a,a∈R
because 3x-y+z=10+a passes P(2,5,-6)
so 3*2-5-6=10+5
so a=-15
so the plane is 3x-y+z=-5
b)
assume the plane:x+ay+bz=c
because the plane passes origin(0,0,0),P(0,2,5)and Q(1,4,0)
so:
0+2*a+5*b=c
1+4*a+0*b=c
0+0*a+0*b=c
so:
a=-1/4,b=1/10,c=0
so the plane is x-1/4*y+1/10*z=0
also:20x-5y+2z=0