how to solve linear equations

Question

3x + 5y = 21 x - y = -1

Answer 1

line 1: x-y=-1 or y=x+1 . . . line 2: 3x+5y=21 . . . line2...3x +5*(x+1)=21 . . . 3x+5x +5=21 . . . 8x=21-5=16 . . . x=2 . . . y=x+1=3

Answer 2

x= -1/18

y= -1/6

here is the reason: 

original equation is 3x+5y=21x-y=-1

so: 3x-21x=-y-5y=-1

18x/18=-1      and  6y/6=-1

which means x=-1/18 and  y=-1/6