Question

     y = x + 1    3x + y = 21

Answer 1

$\left \{ {{y=x+1} \atop {3x+y=21}} \right.\ \ \ Using\ substitution\ method\\\\ 3x+x+1=21\\ 4x=20\ \ |:4\\ x=5\\ y=6$

Answer 2

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

First do this. Change one (or both) of the equations so that the x and y are on one side, and the number is on the other. Make sure either the x or the y is the same in both equations (in this case, y) and then take one away from the other. The y will cancel out so you can work out x.

3x+y=21 -
-x+y=1
-----------
4x = 20
x = 5

Then add the x into one of the two equations to find out y.

3x + y = 21
(3*5) + y = 21
15 + y = 21
y = 6

To check, put both x any y into the other equation.

-5 + 6 = 1

If the answer is correct, you have correctly solved the equations.

x = 5
y = 6

:D