You need to remember that the cross product of perpendicular vectors is zero such that:

`(abari + bbarj)(cbari + dbarj) = ac + bd = 0`

You need to form the vectors `barv_1` and `barv_2` substituting bar a for unit vector bari and bar b for unit vector bar j such that:

`barv_1 = bara+barb`

`barv_2 = 2bara + jbarb`

You need to evaluate cross product `barv_1*barv_2` such that:

`barv_1*barv_2 = 0`

Multiplying the coefficients of like unit vectors yields:

`2*1 + 1*j = 0`

j = -2

**Hence, evaluating the value of j under given conditions yields j=-2.**

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