Question 549837
We know that in any triangle, the sum of any two sides must always be greater than the third side.   
We just need to check if this is true using these sides:)

{{{11+15=26>21}}}
{{{11+21=32>15}}}
{{{15+21=36>11}}}

Since all of the sums are greater than the third side, it is possible to make a triangle with these three sides.