Step-by-step explanation:
Triangle ABC is similar to triangle FED
Using similar triangles theorem:
The ratio of corresponding sides are equal
AC corresponds to FD
AB corresponds to FE
BC corresponds to ED
AC/FD = AB/FE = BC/ED
AC/26 = 5/10 = 12/ED
solving for AC:

5/10 = 12/ED

To check our answer we are asked to use another method
Using pythagoras' theorem:
Hypotenuse² = opposite² + adjacent²
For ABC:
AC² = 12² + 5²