Suppose a gene has three exons, with 570 nucleotides in exon 1, 420 in exon 2, and 810 in exon 3. there is a variant in which exon 2 is also spliced out from the mature mrna. the protein translated from this genetic variant is amino acids long.

Exon one has 570 nucleotides. Exon three has 810 nucleotides. We are not concerned about exon two because it is spliced out in the variant. There are 1380 nucleotides (570+810).  Each amino acid requires three nucleotides. One codon, or three nucleotides, codes for one amino acid. So 1380/3=690 amino acids.