Two waves are traveling in opposite directions on the same string. The displacements caused by the individual waves are given by y1 = (24.0 mm) sin (9.00πt – 1.25πx) and y2 = (35.0 mm) sin (2.88πt + 0.400πx). Note that the phase angles (9.00πt – 1.25πx) and (2.88πt + 0.400πx) are in radians, t is in seconds, and x is in meters. At t = 4.00 s, what is the net displacement (in mm) of the string at

(a) x = 2.16 m and

(b) x = 2.56 m? Be sure to include the algebraic sign (+ or –) with your answers.