The line DB is 8cm which is made up of DE and EB which are equal to each other, thus DE and DB are both 4 cm.
Secondly, since AC and DB are perpendicular, that means angle DEC and angle DEA are ninety degrees.
Thus using the Pythagorean theorem, we can find the length of AE and EC which we can add up to find the length of AC.
By adding AE and EC, we get that the length of AE and EC is 7.47 or as approximated to the nearest tenth 7.5.
Hope that helps!