the sum of all interior angles in a polygon are 180(n - 2) where "n" is how many sides are there in the polygon.
now, this polygon has 7 sides, a Heptagon, so the sum of all its interior angles is 180( 7 - 2) or 180(5) or 900. And we know that angle D is a right angle, thus,
A+B+C+D+E+F+G = 900
120 + B + 148 + 90 + 142 + 130 + 129 = 900
B + 759 = 900
B = 900 - 759.
