Again i don't understand :(

13) x=4, y=0

15) x= -3, y=1

Step-by-step explanation:

13. Label the 2 equations.

y= x-4 (1)

-4x -6y= -16 (2)

From (2): -2x -3y= -8 (3)

(divide by 2 throughout)

Subst. (1) into (3):

-2x -3(x-4)= -8

-2x -3x +12= -8 (expand)

-5x= -8 -12 (simplify, constant to 1 side)

-5x= -20 (simplify)

x= 4 (divide by -5 throughout)

subst. x=4 into (1):

y= 4 -4

y= 0

15. x= y -4 (1)

-2x +3y= 6 (2)

subst. (1) into (2):

-2(y-4) +3y= 6

-2(y) -2(-4)= 6 (expand)

-2y +8= 6

-2y= 6 -8 (bring constant to 1 side)

-2y= -2

y= 1 (divide by -2 throughout)

subst. y=1 into (1):

x= 1 -4

x= -3

use this formula:

p(a\b) = p(a and b)/p(b).

take it from here.