Euclidean Algorithm for GCD of polynomials
Solution 1:
As $(a,b)=(a+n\cdot b,b),$ where $n$ is any integer
$$(2x^2+6x+3,2x+1=((2x+1)x+5x+3,2x+1)=(5x+3,2x+1)$$
Now, $2(5x+3)-5(2x+1)=1\implies(5x+3,2x+1)=1$
Solution 2:
You can observe that if you divide any polynomial then degree of remainder is less than that of divisor . If you divide $2x^2+6x+3$ by 2x+1 the remainder is 13/2 and they also don't have any common constants . So the GCD will be 1.