hi! thanks for confirming that this is the rhombus. i found a picture of it which i've attached below.
to answer this question, let's keep in mind that in any rhombus, the diagonals bisect each other at right angles. this means that each diagonal cuts the other into two equal parts. the diagonals of ABCD are AC and BD, with E serving as the midpoint of both.
E divides AC into two parts: AE and EC. from the above, we know that AE and EC will be equal! if AE = 8, EC also = 8, and AC = 16.
i imagine you don't want to write out a huge paragraph, so here's everything i just said in proof format, as a way to show your work:
STATEMENT /// REASON
1. AE = 8 /// 1. Given
2. AC = BD /// 2. The diagonals of a rhombus bisect each other at right angles
3. AE = EC /// 3. Definition of midpoint
4. EC = 8 /// 4. Substitution (based on given)
5. AE + EC = AC; 8 + 8 = AC; AC = 16 /// 5. Addition
hope this helps and sorry for taking a while :)