Second order maxima?

3.944 m. This can be derived by solving for lambda/a in the equation y=D*tan(theta), where theta = arcsin(m*lambda/a). First we solve for theta1 and lambda/a (0.3422819) with y=1.53 m, D=2.1 m, and m=1. Theta1 = 0.3493444 rad. Then we can compute theta2 with m=2 (0.754005 rad) and solve for the new y. Note the small-angle approximation would have been very inaccurate in this problem.