As per given by the question,
There are given that,
The measure angle is,

And,

Now,
From properties of isoscales triangle,
The two angles opposite to the equal sides are equal.
So,

From the given triangle;

So,

Then,
For the measure first angle;
So,
Put the value of x in given angle,

Hence, the value of x is 16 and the measure of the first base angleangle is 71 degree.