In my derivation I never said to graph the V shape curves. I suggested you sketch them to visualize the solution
y= ItI
The absolute euation may be replaced by two equations , one for t greater than the vertex value , and one equation below the vertex
y=ItI is
y=t for 0<t
y=-t for t<0
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y=I 2t-6 I replace by
y=2t-6 for 3<t
y=6-2t for t<3
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if you sketch, or visualize , the functions you can "see" where ItI > I 2t-6 I
then you can complete the problem
Arthur