Showing that a*a+26 is not prime when a=3 is true but not a math. proof. However it is quite easy: (if I didn't misunderstand) a*a + 26 = a*a -1 + 27= (a-1)*(a+1) + 27
Now, since a is prime, it can not be a mutliplier of 3. Then either (a+1) or (a-1) should be a multiplier of 3. Then (a-1)*(a+1) is multiplier of 3 And (a-1)*(a+1) +27 is also multiplier of 3 Then it's not prime.