Here's the problem:

I already solved the answer for 15.a) which is \(2{\sqrt{38}}\) m, what I need help is with part b.

Here's my work:

but the answer is \({8\sqrt{19}}\) m.

What did I do wrong?

You haven't done anything wrong! You have just calculated ONE SIDE of the square!!.

So, all you have to do is multiply: 4 x 2sqrt(19) =8 sqrt(19).