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Dividing both sides by 4
Now looking at the y=sin(x) graph we have
We can see that the graph has a y value of 0 ever 180 degrees (though it is a bit hard to see as this graph is in radians, this is a known fact) (or 180*k where k is an integer).
So we get (from now on k is an integer)
Hopefully this helps! Let me know if you are unsure still.