Solve x and y:

1) 3xy=9
x+y=4

2) xy=4
x-2y=7

it would be if you told me how to work it out because i am not sure of a method.

1)
3xy=9
x+y=4
substitution method:
x=4-y

3(4-y)y=9
12y-3y²=9
3y² -12y+9=0
y²-4y+3=0

We solve this quadratic equation:
y=[4⁺₋√(16-12)] / 2=(4⁺₋2)/2
We have two solutions:
y₁=1; then x₁=4-y=4-1=3
y₂=3; then x₂=4-3=1

1) we have two solutions:
Solution₁:    x₁=3; y₁=1
Solution₂:    x₂=1; y₂=3.

2)
xy=4
x-2y=7
We solve by substitution method.
x=7+2y
(7+2y)y=4
7y+2y₂=4
2y₂+7y-4=0

We solve this quadratic equation:
y=[-7⁺₋√(49+32)] / 4=(-7⁺₋9)/ 4
We have two solutions:
y₁=-4; then x₁=7+2y=7+2*(-4)=-1
y₂=1/2; then x₂=7+2*(1/2)=7+1=8

2)we have two solutions:
Solution₁:  x₁=-1;  y₁=-4
Solution₂:  x₂=8; y₂=1/2