An object moves at speed v1 with respect to frame S′. Frame S′ moves at speed v2 with respect to frame S (in the same direction as the motion of the object). What is the speed, u, of the object with respect to frame S? Solve this problem (that is, derive the velocity-addition formula) by drawing a Minkowski diagram with frames S and S′, drawing the worldline of the object, and doing some geometry.