Solution:
We know that:
- Interior angles of a triangle sum up to 180°.
![3a + 7u + 9 = 180\°](https://img.qammunity.org/2023/formulas/mathematics/college/7i65ddb430l1tr8fs2jpdjms46dm8ny8xg.png)
This means that:
Let's simplify the equation to find a.
Step-by step calculations:
Open the brackets:
Cancel same terms on both sides:
Simplify the LHS:
Subtract 2a both sides:
![2a - 2a + 29 = 3a - 2a](https://img.qammunity.org/2023/formulas/mathematics/college/qkralwb6a2lmov1n1prm2l0n2n6ov18pox.png)
- =>
![29 = 3a - 2a](https://img.qammunity.org/2023/formulas/mathematics/college/m0dn1xuxh29qnjvd9cbpndjxqkpbpnodqj.png)
Simplify the RHS and change the sides:
- =>
![\bold{a = 29}](https://img.qammunity.org/2023/formulas/mathematics/college/z0wsr7e5yg341zrujvhns6lf2sxc7enmts.png)
Let's substitute the value of a in 3a + 7u + 9 = 180° to find u.
![3a + 7u + 9 = 180\°](https://img.qammunity.org/2023/formulas/mathematics/college/7i65ddb430l1tr8fs2jpdjms46dm8ny8xg.png)
- =>
![3(29) + 7u + 9 = 180\°](https://img.qammunity.org/2023/formulas/mathematics/college/bnmmrow7gokjx6cxrhra779ejazraa0m4b.png)
Simplify the LHS:
- =>
![87 + 9 + 7u = 180](https://img.qammunity.org/2023/formulas/mathematics/college/z7dr35fyfblgyjg8ty4mhffw7chsqwialq.png)
- =>
![96 + 7u = 180](https://img.qammunity.org/2023/formulas/mathematics/college/bwgi2hgx6471u4qyu7vhtqqv1bk5yyyf3u.png)
Subtract 96 both sides:
- =>
![96 - 96 + 7u = 180 - 96](https://img.qammunity.org/2023/formulas/mathematics/college/tgrqpxcxxcpke6nkuba3re3e8n60vd7qwt.png)
- =>
![7u = 84](https://img.qammunity.org/2023/formulas/mathematics/college/r0xeenks7mxgvwtf006ihfg67t0ra7aa7s.png)
Divide 7 both sides:
- =>
![(7u)/(7) = (84)/(7)](https://img.qammunity.org/2023/formulas/mathematics/college/ix7txtq1mqyp74yveamnnhdwaq44n8u7on.png)
- =>
![\bold{u = 12}](https://img.qammunity.org/2023/formulas/mathematics/college/3yaa621ydvadgjtegajufdcgfuhsje875d.png)