∠FDC + ∠CDA = 180^o (Linear pair)

80^o + ∠CDA = 180^o

∠CDA = 100^o

Since, ABCD is a cyclic quadrilateral

∠ADC + ∠ABC = 180^o

100^o + ∠ABC = 180^o

∠ABC = 80^o

Now,

∠ABC + ∠ABF = 180^o (Linear pair)

80^o + x = 180^o

x = 180^o – 80^o

= 100^o