AB and BC are congruent sides as given by the triangle. Their corresponding angles ∠C and ∠A are also congruent.
If ∠C = 51°, then ∠A is also 51°.
Recall that the sum of the interior angle of a triangle is 180°, then we have the following:
[tex]\begin{gathered} ∠A+∠B+∠C=180° \\ 51°+∠B+51°=180° \\ ∠B+102°=180° \\ ∠B=180°-102° \\ ∠B=78° \end{gathered}[/tex]
Therefore, ∠B = 78°.
