Angle within a rectangular-based prism

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I've tried to find solutions to similar problems to see how this question should be solved but I didn't have much luck.

A rectangular-based prism is shown below. I need to determine the size of angle ABC using the vector dot product.

I know how to do the vector dot product for easier problems but this one has me a bit stuck.

Prism

I created a right triangle and worked out that 4^2 + 1^2 = 17^2 so AC (hypotenuse) = √17 and the angle would be arctan(1/4) which is approx. 14 degrees.

I'm just not sure where to take it from here and how to actually get angle ABC. I just need to know the steps required to get me there.

How would you do it?