If it's not what You are looking for type in the equation solver your own equation and let us solve it.
f in (-oo:+oo)
6.21 = f/12.7 // - f/12.7
6.21-(f/12.7) = 0
6.21-0.07874016*f = 0
6.21-0.07874016*f = 0 // - 6.21
-0.07874016*f = -6.21 // : -0.07874016
f = -6.21/(-0.07874016)
f = 78.86699748
f = 78.86699748
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