the solubility of MG(OH)2 is 0.61 g/L

What must be the PH of this solution.

and thx dr. bob for last question

You must mean Mg(OH)2.

Mg(OH)2 ==> Mg^+2 + 2OH^-
If solubility of Mg(OH)2 is 0.61 g/L, convert that to moles/L. moles = grams/molar mass. Call the solubility in moles/L = S

That is Mg(OH)2. Thus (Mg^+2) = S and (OH^-) = 2S. Use that to calculate pOH and pH from that.