Question 1128393
One method for verifying the validity of an argument is the visual technique based on Euler diagrams
This technique is similar to Venn diagrams, in that circles are
used to denote sets, with

-overlap indicating shared elements
-disjoint circles indicating no shared elements
-a circle contained within another circle indicating a subset

in your case:

Following the description, draw a largest circle labeled "vertebrates." Inside that circle, draw a smaller circle labeled "mammals." Inside that circle, draw a smaller circle labeled "dolphins." 

Because the "dolphins" circle is inside the "vertebrates" circle, you can say that all dolphins are vertebrates.

=> the syllogism is valid