Calculus tangent line For some constant c, the line $y=4x+c$ is tangent to the graph of $f(x)=x^2+2$, what is the value of $c$?
I don’t understand how to find the value of c. Because it’s a tangent line I understand they touch at one point. Probably a dumb question, I just don’t understand.

Say you have a tangent at point $T(a,b)$. Then $f'(a) = 4$ and $f(a)=b$ and $b=4a+c$. So we have 
$\bullet \;\; 2a=4\implies a=2$
$\bullet \;\; a^2+2=b\implies b=6$
$\bullet \;\; c=-b+4a\implies c=-2$
