The induced voltage is N(-dΦ/dt)  where N=50 and flux is Φ=BAcos(θ).  B is the field and A is the area.  The trick is to see that because the loop is a square with side length L, that the area is L^2.  For your magnetic flux density values I'm seeing "200 ?t" and 600 ?t"  Am I right in thinking these are microTesla, uT?  I will assume so but if it's something else you'll have to find the answer again So dB/dt = (600e-6-200e-6)/0.4=1e-3.  Now,
80mV=(50)(1e-3)(L^2)cos(30)
L = 1.359m
Thus the perimeter of the square is 4(1.359m) = 5.437m
And the total length is (50)(5.437m) = 271.85m