You haven't given the length of each shark so we can't really calculate the total length of the 7 sharks but I'll just give a strategy to find it.
let [tex]x[/tex] be the length of the blacktop sharks and let [tex]y[/tex] be the length of the tiger sharks. In order to figure out their total length, you can use this formula:
[tex]3x\ +\ 4y\ =total\ length\ of\ sharks[/tex]
