check the picture below.
in a rhombus, the diagonals meet always at right angles, namely the angles at vertex E are all 90°.
also, the diagonals cut each other at equal halves, namely DE = EB, and since we know DE = 8.5, then EB = 8.5..
we also know ∡DBA is 60°, that makes all triangles a 30-60-90 triangle, including the triangle ABE, and using the 30-60-90 rule as you see in the picture, then we get that much.