For manufacturing 'x' units, labor cost is 't(l - 50 - 54x)' and processing cost is 'x²'. The price of each unit is 'p = 800 - 4x²'. Find the values of 'x' for which 'p ≥ 3'.
a) x = 5
b) x = -5
c) x = 4
d) x = -4