"To keep the calculations fairly simple, but still reasonable, we shall model a human leg that is .92m long (from hip joint) by assuming that the upper leg and the lower leg (including the foot) have equal lengths and that each of them is uniform. For a 70kg person, the mass of the upper leg would be 8.60kg, while that of the lower leg (including the foot) would be 5.25kg."

Find the X AND Y COORDINATE of the center of mass of this leg, relative to the hip joint, if it is bent at the knee to form a right angle with the upper leg remaining horizontal.

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Suppose mass of upper leg is m1, lower leg m2

Let length of each leg be L

Centre of mass of upper leg is mid point of upper leg, coordinates (L/2, 0)
Centre of mass of lower leg is mid point of lower leg, coordinates (L, -L/2)

Masses are m1 and m2

Centre of mass of masses m1 and m2 at these two points

X = (m1 x1 + m2 x2)/(m1 + m2) = (m1 L/2 + m2 L)/(m1 + m2)

Y = (m1 y1 + m2 y2)/(m1 + m2) =
(0 m1 -m2 L/2)/(m1 + m2) = -m2 L/2/(m1+m2)

Substitute values of L, m1, m2