hello, this is my first time in this site and it seems great. I need help with subscitution and elimination problems like this problem.
7x88y=25
9x+10y=35

I assume you mean :
7x + 88y = 25
9x + 10y = 35

You approach this as follows : eliminate either x or y, and solve than the remaining unknown.

line 1 : 7x + 88y = 25 --> 7x = 25 - 88y
line 2 : 9x + 10y = 35 --> 9x = 35 - 10y

Now multiply all items in line 1 to get to 63x
And multiply all items in line 2 to get to 63x

Once that is done deduct line 2 from line 1 (so x is eliminated), and calculate the value of y.

Simple, isn't it?

If 7x88y=25 is correct, just do the same trick of eliminating :

7x88y = 25 --> 7x = 25/88y. The rest is similar to above.