+129849 AB + CD = 210 ⇒ CD = 210 - AB
Both triangles ACD and ABC are under the same height ...so....
Area of triangle ACD 3
_________________ = ______
Area of triangle ABC 7
(1/2) h (CD) 3
__________ = ____
(1/2) h ( AB) 7
CD 3
___ = __
AB 7
210 - AB 3
_______ = ____ cross-multiply
AB 7
7( 210 - AB ) = 3(AB) simplify
1470 - 7AB = 3AB add 7AB to both sides
1470 = 10AB divide both sides by 10
147 = AB
