Can someone help me with this question please!!!!!

Solution:
We know that:
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: