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Compare the two triangles  ABC and ADE.    angle A is common.  AD and AB are parallel.   AE and  AC are parallel.  It is known that  a line joining midpoints of two sides is parallel to the third side.     So they are similar triangles.

Since  AD = AB/2 and AE = AC/2,   DE = BC/2   =>  BC = 2 DE = 8 cm