How do I work this out ?

1/2      

Step-by-step explanation:

I enjoy doing these sort of equations. However, I don't remember if it's 2 or 1/2... I haven't done these in forever.

I feel 99.99% certain it's 1/2.

Here's Why: We're trying to figure out how the first triangle (far left) changed (in this case dilated) to "turn into" the triangle on the right. Because of this, I would say 1/2. The triangle has dilated (multiplied itself) by 1/2, leaving a triangle half the size.

EF is half the size the size of BC.