Answer
$y=2x$
Work Step by Step
Lets assume that Equation for tangent line is :
$y=ax+b$
İf this line is tangent to any $f(x)$ at $(x,y)$ point then
$f’(x)=a$
$f\left( x\right) =2x\Rightarrow f'\left( x\right) =2=a$
$y=ax+b\Rightarrow 6=2\times 3+b\Rightarrow b=0$
So the equation will be:
$y=ax+b=2x+0=2x$