We can either convert to standard form first or convert to a common multiplier, kinda like a common denominator. The latter makes more sense if they wanted the result in scientific notation, but let's do it that way anyway.
a)
4.5 × 10⁴ + 3.8 × 10³ = 45 × 10³ + 3.8 × 10³ = 48.8× 10³ = 48,800
Answer: 48,800
b)
4.5 × 10⁴ - 3.8 × 10³ = 45 × 10³ - 3.8 × 10³ = 41.2× 10³ = 41,200
Answer: 41,200
c)
7.2 × 10⁻³ + 6.3 × 10⁻² = 7.2 × 10⁻³ + 63 × 10⁻³ = 70.2 × 10⁻³
= 7.02 × 10⁻² = 0.0702
Answer: 0.0702