Final answer:
The luminance of a lightning flash, given as 70,000,000,000 cd/m2, is expressed in engineering notation as 70 x 10^9 cd/m2.
Step-by-step explanation:
To express the luminance of a lightning flash, which is about 70,000,000,000 cd/m², in engineering notation, we recognize that engineering notation is similar to scientific notation but uses powers of ten that are multiples of three. The value 70,000,000,000 cd/m² in engineering notation can be written as 70 x 10¹ cd/m², because 70,000,000,000 is the same as 70 billion, or 70 times 10 to the power of 9.