Find the Indicated Term Using the Given Information $\mathrm { a } _ { 18 } = 87 , \mathrm { a } _ { 20 } = 99 ; \mathrm { a } _ { 6 }$

Multiple Choice

Find the indicated term using the given information.
- $\mathrm { a } _ { 18 } = 87 , \mathrm { a } _ { 20 } = 99 ; \mathrm { a } _ { 6 }$

A) $- 15$
B) 21
C) 15
D) 6