Show that if $\ 7|5a-2$ then $\ 49|a^2-5a-6\ $ Show that if $\ 7|5a-2$ then  $\ 49|a^2-5a-6\ $ , ($\ a$ is  positive  integer)
My work:
$7|5a-2 \Rightarrow\ 49|35a-14a,49a^2 \Rightarrow\ 49|14a^2+14 \Rightarrow\ 42a^2+42a,49a^2+49a\  \Rightarrow\ 49|7a^2+7a$
And I stopped here!!
I tried to make the coefficient of $a^2$ equal to 1,but no success...

Observe that 
$$2(5a-2)^2-49a^2+7(5a-2)=50a^2-40a+8-49a^2=a^2-5a-6$$
