In the given right triangle base and hypotenuse are given, we have to calculate the length of perpendicular.
Applying Pythagoras theorem in the given triangle which states,
[tex] (Hypotenuse)^{2}=(Base)^{2}+(Perpendicular)^{2} [/tex]
[tex] (21)^{2}=(15)^{2}+(x)^{2} [/tex]
[tex] 441=225+x^{2} [/tex]
[tex] x^{2}=441-225 [/tex]
[tex] x^{2}=216 [/tex]
[tex] x=6\sqrt{6} [/tex]
