in these systems of equations, they give you a point (1, 2) which 1 is x and 2 is y so substitute these numbers instead of x and y to find if they are a solution to it.
y = -2x + 4 (1, 2) 2 = -2(1) + 4 2 = -2 + 4 2 = 2 so this is true.
7x - 2y = 3 (1, 2) 7(1) - 2(2) = 3 7 - 4 = 3 3 = 3 so this is true.