431.2+826.4d=2910.4
431.2 is only the first day therefore it doesn't need the variable because the variable represents the number of days. 826.4 even says "each remaining day" referring to what the variable represents.
A is your answer for the Fatima's age one
C is question 1
X*5-6=8 or 5x-6=8 That is the equation