4x - 2y = 10
y = 2x - 5
Because we have a value for y, we can plug it into the first equation to solve for x.
4x - 2(2x - 5) = 10
Distributive property.
4x - 4x + 10 = 10
Combine like terms.
10 = 10
The equation is true, and so there are infinite solutions, meaning both lines are on top of each-other.