Question: How to write a procedure to determine if values are length of a triangle?

Q3) Write a procedure, isTriangle, to determine if any three given
 numbers represent the lengths of the sides of a triangle.
The procedure should be able to deal with any input.

I have done the following and get an error

isTriangle:=proc(a,b,c::nonnegative and numeric)
if (a^2+b^2)=c^2 then print (a,b,c "represent the sides of a triangle")
end if;
end proc;
Error, unexpected string

when stated 'unexpected string', the cursor is moved to the 
word triangle between the l and e
is there something wrong with my procedure?
many thanks
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