You know that the tangent equation is in the form of y = mx + C where C is a constant.
Since you know the gradient, all you have to do is solve for C by substituting in your values.
Therefore you will have the equation:
$\displaystyle y = (\frac{-4}{25})x + C$
$\displaystyle 0.6 = (\frac{-4}{25}) \times 3 + C$
$\displaystyle 0.6 = -0.48 + C$
$\displaystyle 0.6 + 0.48 = C$
$\displaystyle C = 1.08$
And so your equation will be: $\displaystyle y = (\frac{-4}{25}) x + 1.08$