Question

2x+4y=14.40
2x+4y=14.40-4y
2x=14.40-4y
1x=720-2y... Then I get stuck

Answer 1

So what are your two equations that you started with?

Answer 2

$$
2x+4y=14.40
$$
is one equation
What is the second?

Answer 3

Whenever you have 2 variables, you need 2 equations to solve it.

Same thing if you have 3 variables, you need 3 equations, and so on.

So you need need one more unique equation to solve, involving x and y or just one of them.