matrices help

$2/5a+6 \begin{bmatrix} 3&\\ -2& \\7& \end{bmatrix} = \begin{bmatrix} 15& \\ -10 & \\35 \end{bmatrix}$

we realize that the each element in the matrix was multiplied by 5 so a must be a value that will make 2/5a+6= 5 therefore a must be a negative reciprocal of 2/5 thus we will get -5/2 because that gives us -1 and when you add -1+6=5 which satisifes the equation. 

hope this helped. :) 

[(2/5)a + 6] * 3   = 15

(6/5)a + 18  =   15

(6/5)a  =  - 3

a  =   (-15/6)   =  (-5/2)

Check this in the other two  evaluations 

[ -1 + 6 ]  (-2)  =  [5] (-2)   =  -10

And

(-1 + 6 ] (7)  =  [5](7)  =  35

And Nirvava is correct  !!!!