For (a), only x=1/4 is a solution, and not x=-1/9. When you square both sides of an equation, you are only showing that the results that follow are possible solutions (because squaring only implies the next statement and not the reverse). You need to substitute the possible answers back into the original equation again to check. In this case only x=1/4 works.

For (b), the question is asking for the values of a and b, which are respectively 3 and 6.