Question

X - y = 3
x + 3y = 9

Answer 1

X - y = 3 is the same as x=3+y

3+y+3y = 9

3+4y=9

4y=9-3

4y=6

y=6/4

y=3/2

x-(3/2)=3

x=3+(3/2)

x=6/2+3/2


x=9/2

Answer 2

Solve for one of the variables in the first equation (in this case, I will solve for X):
X = Y + 3

Then use that value of X in the second equation to solve for Y:
X + 3Y = 9
(Y + 3) + 3Y = 9
4Y + 3 = 9
4Y = 6
Y = 1.5

Use the value of Y we just found in the X equation we created:
X = Y + 3
X = 1.5 + 3
X = 4.5

Therefore X = 4.5 or 9/2 and Y = 1.5 or 3/2.