We can already plug in 78 for d in this equation.

78 = 0.05s^2 + 1.1s

After subtracting 78 from both sides, we have:

0.05s^2 + 1.1s - 78 = 0.

Solving this quadratic, we have: s = 30.0000000000000004, -52.00000000000001

The last time I checked, we can’t go negative speed, so s = 30

It is actually 30.000000000000004, but just round it down.