I'm having trouble finding a value of x for f(x) whose tangent line has the slope 4/3. I am not given any information regarding points on the graph.

The following is the original function

$\displaystyle f(x)=3x^2/(2x+1)$What I have done so far is found the derivative and set that equal to 4/3 and then solve for x. This has not worked for me so far. What should I do?