Solve by substitution method?

3x + 6y = 0
3x + 30 = y

3x + 6y = 0
3x = -6y

3x + 30 = y
-6y + 30 = y
30 = y + 6y
7y = 30
y = 30/7

3x + 6y = 0
3x + 6(30/7) = 0
3x + 180/7 = 0
3x = -180/7
x = (-180/7)/3
x = (-180/7)(1/3)
x = -60/7

∴ x = -60/7 , y = 30/7

Your Question

Equation (1) 3x + 6y = 0
Equation (2) 3x + 30 = y <-- Given

replace y into Equation 1

3x + 6(3x + 30) = 0
3x + 18x + 180 = 0
21x = -180
x = -180/21

replace x into given equation to obtain y
y = 3x + 30
y = 3(-180/21) + 30
y = -540/21 + 30

Answers: x = -8.57, y = 4.29

3x+6y=0
3x+30=y

3x = - 6y
-6y + 30 = y
7y = 30
y = 30/7

3x = -180/7
x = - 60/7