Draw your triangles. You will then see that
sinA = -4/5
cosA = 3/5
sinB = -3/7
cosB = -√40/7
Now just plug those into the formula for cos(A-B)
sin A=-4/5;A in Quadrant 4 and csc B=-7/3;B in Quadrant 3.
What is the value of the expression cos(A-B)?
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