As per given by the question,
There are given that,
The measure angle is,
![m\angle ABC=(9x-73)^(\circ)](https://img.qammunity.org/2023/formulas/mathematics/college/arpepmz7gatdyyxx209lzoh054v2i4oyqc.png)
And,
![m\angle ACB=(3x+23)^(\circ)](https://img.qammunity.org/2023/formulas/mathematics/college/lf7dvk36t69ipsbd4jy0v56dufmmaxiz86.png)
Now,
From properties of isoscales triangle,
The two angles opposite to the equal sides are equal.
So,
![\angle B=\angle C](https://img.qammunity.org/2023/formulas/mathematics/college/v82mcyukvmkcecsqsdtz5phq3rj8q5sq6i.png)
From the given triangle;
![\begin{gathered} \angle B=\angle C \\ (9x-73)=(3x+23) \\ 9x-73-3x-23=0 \\ 6x-96 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/h7y33iqcxfgxc7n4zvmiidb27h4bmgljyz.png)
So,
![\begin{gathered} 6x-96=0 \\ 6x=96 \\ x=(96)/(6) \\ =16 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/rj9hi3d2a3sx9epma15ctc9q2czocj1jr7.png)
Then,
For the measure first angle;
So,
Put the value of x in given angle,
![\begin{gathered} m\angle ABC=(9x-73)^(\circ) \\ =(9*16-73) \\ =71^(\circ) \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/jhtpvocv74ja7xw248u3zaj77rtox61er7.png)
Hence, the value of x is 16 and the measure of the first base angleangle is 71 degree.