Well the derivative is defined as the slope of the tangent line, so if you use slope-intercept notation of y=mx+b, then

you already know : y=5x + b for the tangent line.

Because the tangent line is at x=3, then a point on the tangent line is also at x=3. The problem gives you f(3)= -1 so there is a tangent line at (3,-1).

Plug those values into the equation to get b: -1=5(3) + b.

So, -1= 15 + b ........... -16=b!!

So the final equation is y=5x -16.

Glad I could use my calc knowledge somewhere! you can thank me later.....