Because we're given two equations and two unknowns, all we need to do is add the equations to one another. It's helpful to rearrange so that all of the x-values are on top of each other and likewise for the y-values.
x - y = 6
x + y = 4
--------
2x = 10
==>x=5 and y = -1
Now: notice the values that are given. If we have to choose between 3x and pi*x, then for positive values, pi*x is roughly 3.14*x so it will be greater, of course, than 3x. Note that for negative values of x, 3.14*x would be less.
In this case, x is indeed positive, so the
answer is B.
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