Acollection of 54 coins is worth $3.90. the collection contains only nickles and dimes let n represent the number of nickles and let d represent the number of dimes. how many of each type of coin are in the collection

OK. So. we know that there is a total of $3.90
A dime is worth $0.10 and a nickle is worth $0.05
So, yo need to make a simple equation here.
Im not gonna give you the answer, just tell u how to do it. 
Just tell me what you get in the comments. 

Answer

Both are correct.

Explanation
The both equations are the same.
When we say that y=x+10, we can make x the subject of the formula and get;
y=x+10
y-10=x
x=y-10.

If the substitution is done correctly, that is x+10 for y and y-10 for x in the other equation then, both would be correct.