As we can see in this problem, the two sides of the triangle are equal. This means that this triangle is an isosceles triangle. The angles of the triangle on its base (the base is the side with a length of 5.1) are of equal value. Hence, we have base angles equal to 39°. We are solving for the value of x. The interior angle of the triangle is equal to 180°. To solve x, we have
[tex]\begin{gathered} 39^{\circ}+39^{\circ}+x=180^{\circ} \\ x=180^{\circ}-78^{\circ} \\ x=102^{\circ} \end{gathered}[/tex]Answer: x = 102°