Please help!!
AD¯¯¯¯¯ is an altitude, and ∠BAC is a right angle. --> given
∠ADB and ∠ADC are right angles. --> Definition of altitude
∠BAC≅∠BDA -> ???
∠BAC≅∠ADC

∠B≅∠B ---> AA Similarity Postulate
∠C≅∠C
ABBD=CBAB ---> ???
(AB)2=(CB)(BD) Cross multiply and simplify.
ACDC=CBAC Polygon Similarity Postulate
(AC)2=(CB)(DC) Cross multiply and simplify.
(AB)2+(AC)2=(AB)2+(CB)(DC) Addition Property of Equality
(AB)2+(AC)2=(CB)(BD)+(CB)(DC) Substitution Property of Equality
(AB)2+(AC)2=(CB)(BD+DC) ---> ???
BD+DC=CB Segment Addition Postulate
(AB)2+(AC)2=(CB)2 Substitution Property of Equality