Answer:
Show ΔBCD ≅ ΔGFE, so ∠C ≅ ∠F. Base angle of an isosceles triangle are congruent, so ΔACF is isosceles.
Explanation:
Informally, subtract DE from CE and DF. This will show CD ≅ EF.
Then ΔBCD ≅ ΔGFE by the HL theorem for right triangles.
Corresponding parts of congruent triangles are congruent, namely the angles C and F.
Since base angles of ΔACF are congruent, it is isosceles.