1. The problem statement, all variables and given/known data Statement: Find the principal axes of the section shown: The origin is on the top left corner. 2. Relevant equations Centroid equations: Second moments of area: Mohr's Circle for I equations: Coordinates Centre Angle from principal axes: 3. The attempt at a solution Finding the centroids: Finding Iy, Iz, and Iyz (Finding Iyz is where I'm having some difficulties): THE PROFESSOR-PROVIDED SOLUTION FOR Iyz (I don't know how to determine whether or not y1, z1, y2, z2 are positive or negative) (My prof's answer was Iyz = -1.188×106mm4) Here is his provided solution: I know where his values come from, I just need a surefire trick on determining the sign of y1, z1, y2, z2. Mohr's Circle for I (I know I'm missing the axis labels. x axis is Is, y axis is Ist) Each point is multiplied by 10-6: Centre: Angle of Principal axes: TL;DR Need a trick on determining the sign of y1, z1, y2, z2 from the Iyz equation.