If we decrease half of a number by some value, we'd have to add that value and multiply the new number by 2 to take into account the 'Half of a number' part.
So in this problem, what I would use is:
(1/2)x - 19.7 = -4.1
Adding 19.7:
(1/2)x = 15.6
Multiply by 2:
x = 31.2
x = 31.2
