Concept:Use the area formula of a right triangle and factorization to find the two perpendicular sides, then apply Pythagoras theorem.Explanation:Let the shorter side be x cm and the longer side be (x+23) cm.Area of right triangle = 21×base×height.So, 21×x×(x+23)=210.This gives x(x+23)=420.Thus, x2+23x−420=0.Factorizing: (x−12)(x+35)=0.So, x=12 cm, since x=−35 is not possible.Hence, the two sides are 12 cm and 35 cm.Using Pythagoras theorem: H2=122+352=144+1225=1369.Therefore, H=37 cm.Perimeter =12+35+37=84 cm.Answer:84 cm, i.e., Option A.