Question

I really need help oof-

Answer 1

From the given info (and the linked question) we find

$\cos\alpha=-\dfrac5{13}$

$\sin\alpha=\dfrac{12}{13}$

$\sin\beta=-\dfrac45$

Then using the angle-sum identity for cosine, we have

$\cos(\alpha-\beta)=\cos\alpha\cos\beta+\sin\alpha\sin\beta$

$\cos(\alpha-\beta)=\left(-\dfrac5{13}\right)\dfrac35+\dfrac{12}{13}\left(-\dfrac45\right)=-\dfrac{63}{65}$