In a triangle, the sum of the lengths of any two sides must be greater then the length of the third side. If you can show any two segments, the sum of whose lengths is less than the length of the third segment, that cannot form a triangle.
In choices A, B, and D, there is at least one sum of the lengths of two segments that is less then the length of the third segment. That shows that choices A, B, and D cannot form triangles.
You cannot divide first because in equations you need to isolate the variable, which means subtracting first. The outcome will be different if you change the order.
