Here's the formula for molarity:
#"molarity" = "number of moles"/"volume (L)"#
To find the molarity of #Sr(OH)_2#, we'll need to find the number of moles of #Sr(OH)_2#. The question also already tells us its volume (#"20.0 mL"#, which is #"0.02 L"#), so we won't need to find that.
From the chemical formula, we know that #Sr(OH)_2# is a diprotic base because there are #2# #OH# ions.
This just means that #Sr(OH)_2# is capable of accepting #2# #H^+# ions in a neutralization reaction.
Knowing this, we can say that, for every #1# mole of #Sr(OH)_2# that reacts, #2# moles of the monoprotic #HCl# will have to react.
From this, we can find the number of moles of #Sr(OH)_2#, because it will be two times the number of moles of #HCl#.
The number of moles of #HCl# can be found by multiplying its molarity (AKA number of moles per litre) with its volume (the number of litres there are).
#"moles of HCl" = "0.05 M" xx "25.0 mL" = "0.05 M" xx "0.025 L"#
#"= 0.00125 moles"#
Therefore, #0.00125-:2 = 0.000625# moles of #Sr(OH)_2# must have reacted.
Now, we can just plug all of these values into the formula for molarity:
#"molarity" = "number of moles"/"volume (L)"#
#"molarity" = "0.000625 moles"/"0.02 L" = "0.03125 M"#
The molarity of #HCl# that was given to us in the question only had #1# significant figure, so our answer should also only have #1# significant figure.
That makes it #"0.03 M"#.
