Answer:
The correct option is (d)-He wrote commutative instead of associative in Step 2.
Step-by-step explanation:
Commutative property: 
Associative property:
Now consider the provided expression.
(6.2 − 1.6) − 4.4 + 7.8
Step 1
(6.2 − 1.6) − 4.4 + 7.8
Step 2 Use Associative property
6.2 + (−1.6 − 4.4) + 7.8
Step 3: Simplify
6.2 − 6 + 7.8
Step 4: Use commutative property
14 − 6
Step 5: Simplify
14 − 6 = 8
Hence, the wrong step is step 2. He wrote commutative instead of associative in Step 2.
Therefore, the correct option is (d)-He wrote commutative instead of associative in Step 2.
The plane PRS passes through the points P, R and S. So it contains the line RS. Also the plane QRS passes through the points Q, R and S. So it contains the line RS as well. Since both the planes contain the line RS, the line RS must be the intersection of plane PRS and QRS
Think of 6y as
y + y + y + y + y + y
Now subtract a y from that.
y + y + y + y + y + y - y = y + y + y + y + y = 5y
He was born on July 2, 1924 and he died on June 9, 1972
~Hope this helped