How to solve these questions using substitution

For question 6
Make x for first one subject of formulae
then it becomes
x=-2y
Substitution means replace your answer in the second part
since x=-2y
3(-2y) +4y = 4
-6y+4y=4
-2y=4
y= 4/-2
y=-2

Now to find x,replace the answer for y in one in the equation
so i replace y=-2 in the first part
x=-2y
x= -2(-2)
x=4
Therefore x=4 and y=-2
To check your answer replace x and y by the values you got and check if you get same answer.