Which of the following is a foci of x2/9+y2/4=1 . (√13, 0) (√5, 0) (0, √5)

For this problem, you first plug y=3x-14 in to x+3y=4

The equation will thin look like this:
x+3(3x-14)-14

since the value of y is equal to 3x-14, that means that 3y=3(3x-14)

then you distribute 3 into 3x-14
it will look like this:
3*(3x)+3*(-14)
(9x)+(-42)
9x-42

now you should have written x+9x-42=14

next, combine like terms
x+9x-42=14

10x-42=14
    +42 +42
10x=56

now, you divide 56 by 10
10x=56
10    10

x=5.6

okay, now that x has been solved for, we now need to find the value of y
the simplest way to go about this is to choose the equation that already has y by its-self.
y=3x-14
plug 5.6 in to x
y=3(5.6)-14
now solve for y

y=(16.8)-14
y=2.8

FINAL ANSWER
y=2.8
x=5.6

if you want to check your work, you can plug x and y into either equation

x+3y=14
(5.6)+3(2.8)=14
5.6+8.4=14
14=14

y=3x-14
(2.8)=3(5.6)-14
2.8=16.8-14
2.8=2.8

I hope this helps.