Here, Assume, the parents are normal means, they aren't diagnosed with that disease. Let the Genotype of Female parent is XcX, it means, she is carrier of that disease. and Let male parent has genotype XY, it means, he is normal.
Cross: Xc X × X Y
F1 Offspring: Xc X, Xc Y, XX, XY
Here, Xc Y would be the child with that disease. As "Cystic Fibrosis" is a recessive disease, it will show in a male when there is 1 copy on either gene, but in female, she must have both genes.
Genotype of Child with Cystic Fibrosis: Xc Y
Hope this helps!