Answer:
![B(a)=(a)/(6)-9](https://img.qammunity.org/2021/formulas/mathematics/middle-school/gq862y7qy678pz2iw9w71cy6p4xjkl70tz.png)
Explanation:
see the attached figure , to better understand the problem
we have
![A(b)=12((b+9)/(2))](https://img.qammunity.org/2021/formulas/mathematics/middle-school/gmo8cj7hj2n2w2isdlkrw0d43z3n7tzyop.png)
where
A(b) ---> is the trapezoid's area
b ---> is the other base value
Solve the equation for b
That means ----> isolate the variable b
Divide 12 by 2 right side
![A=6(b+9)](https://img.qammunity.org/2021/formulas/mathematics/middle-school/t8pe2rhw2wdm50mhfx29267za7xwwsbxa7.png)
Divide by 6 both sides
![(A)/(6)=b+9](https://img.qammunity.org/2021/formulas/mathematics/middle-school/7o0twm3y3xkzftk81q128oegh8rkwar4ow.png)
subtract 9 both sides
![(A)/(6)-9=b](https://img.qammunity.org/2021/formulas/mathematics/middle-school/5xazctz4sek3yqbla5nx2w81ugz5yqsovy.png)
Rewrite
![b=(A)/(6)-9](https://img.qammunity.org/2021/formulas/mathematics/middle-school/pn1cd23uyzhd0rj0lnadu0rma8gl1s70s5.png)
Convert to function notation
![B(a)=(a)/(6)-9](https://img.qammunity.org/2021/formulas/mathematics/middle-school/gq862y7qy678pz2iw9w71cy6p4xjkl70tz.png)