mathematically similar

Nov 2009
Triangle ABC has lengths of 3+x, 6 and (unknown).
Triangle DBE is within this triangle and it has lengths 3, 4 and (unknown).

Although these are mathematically similar, the two unknowns (DE and AC) are not parallel.

Work out x

I don't know how to approach this because I can't use the simple formula of that over that equals this over this due to the unknowns not being parallel