Simple answer to the equation without the circumstances:
3.1x - 1.6x = 35 + 10
1.5x = 45
x = 30
First you would find that not all the units are the same in this problem, so you convert yards to feet. So 4 yards = 12 feet.
The proportion you would make would be x/12 = 2/1
The cross product of two vectors gives a third vector

that is orthogonal to the first two.

Normalize this vector by dividing it by its norm:

To get another vector orthogonal to the first two, you can just change the sign and use

.