Calculus help please

Question

The side of a square is measured to be 12 ft with a possible error of ±0.1 ft. Use differentials to estimate the error in the calculated area. Include units in your answer. 

Answer 1

Area A = x². We will replace δA by dA: δA ≈ dA = A'dx = 2x·dx = 2·12 ft·(±0.1 ft) = ±2.4 ft²