Answer
$b=3$
Work Step by Step
$Given,$ $\sqrt (10b+6)=6 $
Squaring,we get:
$10b+6=36$
$10b+6-6=36-6$
$10b=30$
$b=30\div10=3$
We need to check by putting the obtained value in the original equation :
$L.H.S=\sqrt(10X3+6)=\sqrt 36=6=R.H.S$
Hence,$b=3$ is a valid solution