The angle you want - call it <em>θ</em> - is such that
tan(<em>θ</em>) = FC / AC
Find the length of the diagonal AC, i.e. a diagonal of the rectangle ABCD. ABC forms a right triangle with legs AB = 70 and BC = 50, so
AC² = AB² + BC²
→ AC = √(70² + 50²) = 10 √74
Find FC using the given angle of the sloping face:
tan(30º) = FC / BC
→ 1/√3 = FC / 50
→ FC = 50/√3
Now solve for <em>θ</em> :
tan(<em>θ</em>) = (50/√3) / (10 √74)
→ tan(<em>θ</em>) = 5/√222
→ <em>θ</em> ≈ 18.6º
