It seems like you're asking for the calculation of the length of line segment $EF$ or something similar. However, I'll need more details to provide an accurate calculation. Here are some possible contexts:

1. Given Points $E(x_1, y_1)$ and $F(x_2, y_2)$ on a Cartesian plane: You might be looking for the distance between these two points.
2. In Geometry, $EF$ could be a side of a triangle or another polygon: You might want to calculate the length of a side based on other given sides or angles.
3. Vector $\mathbf{EF}$: You might be calculating the magnitude of the vector from $E$ to $F$.
4. A line segment in a coordinate system: You might be applying the distance formula.
5. $EF$ in a triangle: It might relate to the sine or cosine rule.

Could you please clarify the context or provide additional information?