Question

Solve the system of linear equations below. y = -2x + 19 y = x + 7

Answer 1

y = -2x + 19
y = x + 7

Just use substitution where you take a variable and plug in an equivalent. It plainly says that y is equal to -2x + 19, so use that for y in the second equation.
-2x + 19 = x + 7
-3x = -12
x = 4
Then, plug this into the equation.
y = x + 7
y = 4 + 7
y = 11

Answer 2

Yup. So if a=b and a=c. Then b=c. That's the key. :D