So there are two triangles here: Smaller one (ADE) and bigger one (ABC) and they both are similar.
So you can use proportions here.
AB / AC = AD / AE
AB = AD + DB = 6 + 1 = 7
AC = AE + 3
AD = 6
So plug in these values:
AB / AC = AD / AE becomes
7 / (AE + 3) = 6 / AE
Now do the cross multiply:
7 AE = 6 (AE + 3)
Now solve for AE:
7AE = 6AE + 18
AE = 18
