.
Wording is very poor in this post,
but, straining my guessing skills, I can imagine
+--------------------------------------------------------------------+
| that were there n groups last year with x students in each, |
| with nx = 105, |
| and that there are (n+2) groups this year with the same number |
| x of students in each group, with (n+2)*x = 175. |
+--------------------------------------------------------------------+
If so, then I have two equation
nx = 105 (1)
and
(n+2)x = 175. (2)
Subtracting equation (1) from equation (2), I get
2x = 175 - 105 = 70; hence x = 70/2 = 35
and the number of groups last year was n= 105/35 = 3. ANSWER
Solved.