Question 1106931
.
<U>Answer</U>.  b = 0.



<U>Solution</U>


<pre>
    x-coordinate of the vertex is {{{-b/(2a)}}}.


    The fact that the vertex lies at the y-axis means that x-coordinate of the vertex is zero,  or, in other words, {{{-b/(2a)}}} = 0.


    It implies  b = 0.
</pre>

Solved.