Question 520900
If Mindy has 2 more than 3 times what Fiona does, substitute in x.

Let x= # Fiona's rocks
Let 3x+2= # of Mindy's rocks ((3x is three times the number of Fiona's rocks, then +2 is the two more.)

Together, they have 54 rocks. So Fiona's rocks plus Mindy's rocks. 

(x) + (3x+2) = 54
add like symbols

4x + 2 = 54
    -2   -2
subtract 2 from both sides

4x = 52
---  ---
4     4

Divide both sides by 4

x=13

Since x is the number of rocks Fiona has, Fiona has 13 rocks.