It is finding scalar projection of b onto a $$a = \langle-5, 12\rangle,\ b = \langle4, 6\rangle$$ $\operatorname{comp}_ba = |b|\cos \theta = \frac{a\cdot b}{|a|} = \frac{-20 + 24}{13} = \frac{4}{13}$
Well, this is how I solved it and I don't think there is no mistakes here. But the text book's solution says it is 4.
Maybe I am misinterpreting basic concept of inner product. Can someone tell me what it is that I am misunderstanding?
It's all correct, there's just a little mistake in the calculation of the dot product $$\frac{a\cdot b}{|a|} = \frac{-20\color{red}{+72}}{13} = \frac{52}{13} = 4$$