John has six more nickels than dimes in his pocket for a total of 2.25 write an equation to determine the number of dimes, x in his pocket

You have to use let statements:
Let x = the number of dimes
Let x+6 = the number of nickles

We know that nickles are represented as $0.05 and dimes $0.10...
Together they make $2.25 right? So you have to add them.

0.05(x+6) + 0.10(x) = 2.25    => This is the equation the problem is asking for.

If they ask you how many nickels and dimes does John have in this pocket, then you have to solve for x, like this:
0.05x + 0.3 + 0.10x = 2.25       In this step you apply the ditributive property.
0.15x + 0.3 = 2.25                    In this step you combine like terms
0.15x = 2.25 - 0.3                     In this step you moved to +0.3 to the other side, but remember to switch the sign!
0.15x = 1.95                             In this step you subtract 2.25 and 0.3
x = (1.95) / (0.15)                     In this step you divide 0.15 in the right because in the left side of the equals sign it was multiplying. You get x by itslef.
x = 13                                       Latly, you divide and you get 13 as an answer

Now that the value of x is 13. You plug this number into the let statements.
We had:
x and x+6 which would be
x = 13
x+6 = 13+6 = 19
Which means that John has 13 dimes and 19 nickels in his pocket.