Shawn bought boxes of thank you cards and get well cards. The thank you cards came in boxes of 7 and the get well cards came in boxes of 9.

OK...let's call the number of boxes he bought holding 7 cards, x. And let's call the number of boxes he bought holding 9 cards, y.

So we have

x + y = 17    (A)

And , the number of cards in each box type, times the number it holds added together = 135 

So we have

7x + 9y = 135   and

x +  y   =   17      By multiplying the bottom equation on both sides by -7, we can "eliminate" x. So we have

7x + 9y =    135 

-7x - 7y =  -119    And we get

2y = 16   So

y  = 8      And substituting this into (A), we have

x + 8 = 17   

So x  = 9

So he bought 9 boxes of 7 cards and 8 boxes of 9 cards = 135 cards

That is too much thinking for me Phill lol