Solution:
We know that:
- Interior angles of a triangle sum up to 180°.

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:

- =>

Simplify the RHS and change the sides:
- =>

Let's substitute the value of a in 3a + 7u + 9 = 180° to find u.

- =>

Simplify the LHS:
- =>

- =>

Subtract 96 both sides:
- =>

- =>

Divide 7 both sides:
- =>

- =>
