Question

How many solutions for each system

Answer 1

Infinitely many is the first one, because you can convert one equation into another one (3x-3y=-18--->  x-y=-6, ---->y=x+6)

second one is one solution, because
y=-4x+11 and y=11+6x,--->-4x+11=11+6x, -4x-6x=0, x=0

third one
y =-2x+5 and y=-7-2x ,-2x+5=-7-2x, --->5=-7 false , no solutions