help me thanks you friend

Dik:p=20 cm,r1=7 cm,r2=5 cm
Dit:d=?
jwb: d=√p^2-(r1+r2)^2
=√20^2-(7+5)^2
=√400-12^2
=√400-144=√256 cm
Jgn lupa Terima Kasih ya,dan jawaban terbaik jika membantu