Slope of roof (pitch) = tan^-1(1.5/6) = 0.244978663 radians = 14.03624347 degrees.

b = 6/cos14.04^o = 6.184658438 = 6185 mm to the nearest mm.

To calculate a first find the angles in the triangle formed by the overhang, the wall of the house and the rays of the sun. These angles in degrees are: 75.96375653, 8, 96.03624347 ( 1.325817664, 0.13962634, 1.67614865 radians). Using the Sine Rule

a = 5.5 sin8
sin96
= 0.769719708 = 770 mm

to the nearest mm.