You will estimate the quantity of DNA present in the pUC18 DNA prep by gel band brightness. Compare your linear plasmid band in the HindIII digest lane to the λ-HindIII marker. If, for example, your linearized plasmid is half as bright as the 4, 361 bp band of the marker, then your approximate DNA quantity is 90/2 or 45 ng. You loaded 10 µl of the HindIII digest onto the gel so your original concentration of DNA in the digest is 45 ng/10 µl or 4.5 ng/µl. You put 10 µl of DNA in your digest. All together then, you have 4.5 ng/µl x 10 µl or 45 ng of DNA in the digest.