18 Here's a geometrical method.
Begin by finding the length of BE.
Let BE = 4L, then BA = 5L, but the figure is a rhombus, so 4L + 2 = 5L, so L = 2 in which case, BE = 8.
Now, take a line through E perpendicular to AB and extend it to the other side of AB to a point E* so that E and E* are equidistant from the line AB.
CE* then cuts AB at the required point P, (for minimum CP + PE).
To see that this is the case, see that PE = PE* so that CP + PE = CP + PE* = CE*, and that if P moves either way along AB, then CP + PE will become bigger.
(Alternatively the line perpendicular to AB could be taken through C, extended to C*. C*E (equal in length to CE*) then gets you the same point P as earlier).
The length of CE* can be found by some simple trig, tanB in the smaller triangle at B followed by the cosine rule in the triangle CEE*.
I got the answer as 2sqrt(745)/5, approx 10.9179.
Tiggsy