Answer:
![36 + h = 45](https://img.qammunity.org/2021/formulas/mathematics/high-school/4f16okmj8oq56u4kokohfy115461ke4kwo.png)
![h = 9](https://img.qammunity.org/2021/formulas/mathematics/college/pha3o7hbwel9teshmlw3cm1agbv4xdqj5t.png)
Explanation:
Given
![Total = 45](https://img.qammunity.org/2021/formulas/mathematics/high-school/aecnt46fids7ujbd51zoeaq877v0d2joi6.png)
![Fall = 12](https://img.qammunity.org/2021/formulas/mathematics/high-school/7sjlg1cai5wwanxxaf28b41clmzadqvmpk.png)
![Spring = 2 * Fall](https://img.qammunity.org/2021/formulas/mathematics/high-school/6lzwgrgu6ozs3ol2wifl7hkkavlknlm41w.png)
Required
Determine the equation to find the hours (h) left
The sum of the hours spent during the fall, spring and the hours left must be equal to the total hours.
In other word;
![Fall + Spring + h = Total](https://img.qammunity.org/2021/formulas/mathematics/high-school/ne948lnqf7ezmsbwf3en6a60cfuc0dtnte.png)
Substitute 2 * Fall for Spring
![Fall + 2 * Fall + h =Total](https://img.qammunity.org/2021/formulas/mathematics/high-school/anezhlfcj6rtxc3knzsecqon5oj8q65kgu.png)
Substitute values for Fall and Total
![12 + 2 * 12 + h = 45](https://img.qammunity.org/2021/formulas/mathematics/high-school/r3lno2q1y7z3fp4m1tj8crwxxeg1i2n159.png)
![12 + 24 + h = 45](https://img.qammunity.org/2021/formulas/mathematics/high-school/y2m1zvpv3mut2pdyhh3d9xhk37br6kcajm.png)
![36 + h = 45](https://img.qammunity.org/2021/formulas/mathematics/high-school/4f16okmj8oq56u4kokohfy115461ke4kwo.png)
The above represents the equation,
Solving further for h
![h = 45 - 36](https://img.qammunity.org/2021/formulas/mathematics/high-school/5jte4e50zufoy9bfh26wm6wiujc0hr54kk.png)
![h = 9](https://img.qammunity.org/2021/formulas/mathematics/college/pha3o7hbwel9teshmlw3cm1agbv4xdqj5t.png)
Hence, there are 9 hours left