Please those who are expert with math, help me.

20

Step-by-step explanation:

opposite formula = - ( x )

here x is -20

- ( -20 )

20

+ 20

Step-by-step explanation:

Opposite of - 20

Multiply - 1 with - 20

= - 1 * - 20

= 20

= + 20

Note : -

- 20 is a negative number.

Opposite of negative number ( - ) is positive number ( + ).

Similarly,

Opposite of positive number ( + ) is negative number ( - ).

x = -3

y = -18

step-by-step explanation:

to solve by substitution, substitute one variable and solve for the other. then substitute again to find the remaining variable.

x-1/3(3x-9) = -3 since y=3x-9 substittute y.

x-1/3(3x-9)=-3

x + x +3 = -3

2x+3=-3

2x+3-3= -3-3

2x=-6

x=-3

substitute again into y=3x-9

y=3(-3)-9

y=-9-9

y=-18