Can it be done? Yes.

a( -1, 5 )^T + 2( 3x, 10 )^T = ( 8, 25 )^T
 ( -a, 5a)^T +  ( 6x, 20 )^T = ( 8, 25 )^T
         ( 6x-a, 5a+20 )^T  = ( 8, 25 )^T

Equating corresponding elements:

6x-a  =  8
5a+20 = 25
   5a = 5
    a = 1

6x-1  = 8
6x    = 9
    x = 9/6 
    x = 3/2

Scaling with values less than One

In the definition of scaling:

a( x, y )^T = ( ax, ay )^T

it is OK if a is smaller than one. For example:

0.5( x, y )^T  =  ( 0.5x, 0.5y )^T

Sometimes this is written as:

( x, y )^T
          =  ( x/2, y/2 )^T  
   2

(Review: ) What is the length of (1,0)^T ?