Answer:
EF is 4 units.
Explanation:
It is given that the two triangles
are congruent to each other i.e.

Coordinates are given as:
A(-1,1)
B(2,4)
C(3,1)
As per the property of congruence, all the corresponding sides and angles of the two triangles are equal to each other.
i.e. AB = ED
BC = DF
AC = EF
We are given with the coordinates of points A, B and C here from which we can easily calculate the side AC using the distance formula.

Where
are the coordinates of the two points whose distance is to be calculated.
So,

Hence, AC = EF = 4 units