You did not provide us with equations to select.
Find the slope m.
m = (1 - 2)/(3 - (-1))
m = -1/(3 + 1)
m = -1/4
Use the slope and one of the points and plug into the point-slope formula.
y - 1 = (-1/4)(x - 3)
Isolate y.
y - 1 = (-1/4)x + (3/4)
y = (-1/4)x + (3/4) + 1
y = (-1/4)x + (7/4)
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The answer is 6. Hopefully this helped!!
I'm going to out on a limb and say 16 in.
4 pieces * 4 in. = 16