value bet river nl 50

[ScarfaceGER]

hi

stats: 43/o/1.22 flop agg: 3.5 and went to sd 33% over 70 hands

OnGame's Room No-Limit Hold'em, $1 BB (5 handed) by Hand Converter CG

saw flop|<font color="#C00000">saw showdown</font>

Hero ($75.25)
Button ($45.15)
SB ($54.70)
BB ($51.70)
UTG ($45.65)

Preflop: Hero is MP with K[img]/images/graemlins/heart.gif[/img], A[img]/images/graemlins/heart.gif[/img].
UTG folds, <font color="#CC3333">Hero raises to $2</font>, Button calls $2, SB folds, BB folds.

Flop: ($4.75) 9[img]/images/graemlins/heart.gif[/img], 5[img]/images/graemlins/diamond.gif[/img], K[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(2 players)</font>
Hero checks, <font color="#CC3333">Button bets $3</font>, Hero calls $3.

Turn: ($10.75) 4[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(2 players)</font>
Hero checks, <font color="#CC3333">Button bets $3.5</font>, <font color="#CC3333">Hero raises to $10</font>, Button calls $6.50.

River: ($30.75) A[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">Hero bets $ ???</font>

board is dry and villain is agg on the flop so I dicide to slowplay tptk.
1. Is it ok to check the turn again?
2. what amount should i bet on river?