2x - y = 10
y = -x - 1
solve the system of equations

(9, 8)
(3, -4)
no solution

You can plug -x - 1 for y in the first equation

2x - (-x - 1) = 10

2x + x + 1 = 10

Combine

3x = 9

Divide

x = 3

Plug 3 for x in any equation ( I'll use th second equation )

y = - (3) - 1

y = - 3 - 1 = -4

x = 3 ; y = -4

(3, -4)

2x - y = 10      (a)

y = -x - 1            (b)

Substitute y = - x - 1 into equation (a) 

2x - y = 10

2x - (-x - 1) = 10

2x + x + 1 = 10

3x + 1 = 10

3x = 10 - 1

3x = 9

x = 9/3 = 3.             x = 3 

Recall, y = -x - 1 = -3 - 1 = -4

Hence solution (x, y) = (3, -4)

Dis a correct answer