Following pls find my worked solution:
Area of triangle = (B x H) / 2
in given figure, label triangle A (90 cm2), B (70 cm2), C (30 cm2), D (shaded).
Area of triangles (A + C) = [B x (sum of perpendicular heights of A,C)] / 2
= (B x L) /2 = (1/2) of Rectangle (since B is common and sum of perpendicular heights is L(ength))
= 90 + 30 = 120 cm2
Hence, shaded area (D) = 120 - 70 = 50 cm2
Trust this helps.
Do let me know if this is different from your Answerkey or if there's further clarification.