Total number of questions = 35 (given)
Let there be x questions which have 7 points.
So, number of questions with 2 points = 35 - x

Since 1 7-point question carries 7 points, x questions would carry
7*x points.

Similarly, since 1 2-point question carries 2 points, (35 - x) such questions 
would carry (35 - x)*2 points.

Total nummber of points in the test = 100

So, 

Expanding the parentheses,



Simplifying
 or 

Number of 7 point questions = 6
Number of 2 point questions = 35 - 6 = 29

Check: Total for 7 point questions = 6*7 = 42
Total for 2 point questions = 29*2 = 58
Adding the two, total for the test = 42 + 58 = 100. Correct.

Hope you got it now.

:)