The Sun (2.0 × 10³⁰ kg) and Mars (6.4 × 10²³ kg) are 2.28 × 10¹¹ m apart…

Step 1. (a) $F = 6.67 \times 10^{-11}\ \text{N m}^2/\text{kg}^2 \times 2.0 \times 10^{30}\ \text{kg} \times \dfrac{6.4 \times 10^{23}\ \text{kg}}{(2.28 \times 10^{11}\ \text{m})^2} = 1.64 \times 10^{21}\ \text{N}$.

Step 2. (b) $a = \dfrac{1.64 \times 10^{21}\ \text{N}}{6.4 \times 10^{23}\ \text{kg}} = 2.57 \times 10^{-3}\ \text{m/s}^2$.

Step 3. (c) $a = \dfrac{1.64 \times 10^{21}\ \text{N}}{2.0 \times 10^{30}\ \text{kg}} = 8.21 \times 10^{-10}\ \text{m/s}^2$.