I don't know if this helps but an equivalent rate is:Rates that represent the same comparison; for example 90km
2h Fraction<------
and 45km/h Confusing i know
To find the final term to compete the square you need to divide the 'x' term by 2 then square it

- equivalent equation

To find the gradient of the tangent, we must first differentiate the function.

The gradient at x = 0 is given by evaluating f'(0).

The derivative of the function at this point is negative, which tells us <em>the function is decreasing at that point</em>.
The tangent to the line is a straight line, so we will have a linear equation of the form y = mx + c. We know the gradient, m, is equal to -1, so

Now we need to substitute a point on the tangent into this equation to find c. We know a point when x = 0 lies on here. To find the y-coordinate of this point we need to evaluate f(0).

So the point (0, -1) lies on the tangent. Substituting into the tangent equation:
So the eastbound train travels at 90 miles per hour in the complete opposite direction to the westbound equivalent, which travels at 80 miles per hour. Therefore, per hour, they get 170 miles away from each other. Since we are working out how long it would take for them to be 204 miles apart:
204 / 170 = 1.2 Hours, or 1 hour and 12 minutes.
Hope I helped!