Determine the equations of the tangent lines to the graph of

f(x)=3x(5x^2+1)

that are parallel to the line

y=8x+9

This is what I have so far: d/dx(3x(5x^2+1))=45x^2+3
45x^2+3=8
x=-1/3, x=1/3
When I sub this into my formula I get f=-14/9 and f=14/9. does this seem right?