First, this has nothing to do with "Linear and Abstract Algebra" and does not belong in this category.
Second, draw horizontal and vertical lines making a right triangle with as hypotenuse. The vertical side is a "near side" to that 75 degree angle and will have magnitude . In order to support the milk carton, we must have . Solve that for .
That will give the force in "kg" which is really a mass. To get the force in Newtons, multiply by g= 9.81[/tex].