Simple.

Accln is along force ( magnetic ) F.

F is perpendicular to both v and B.  [ F = q ( V X B ) ]

so Accln a is perpendicular to B.

so         a.B = 0 

( ti+7j.) . ( (7i-3j)x10 ^-3 ) = 0

( t i  + 7 j ) . ( 7 i  - 3 j  )  = 0

7 t   - 21 = 0

 hence   t = 3

Rate me if it helps.