Why does this Trigonometry math problem simplify like this

Why does
64 = 36 + 49 - 2(6)(7) cos A
simplify to
cos A = (36 + 49 -64)/(2(6)(7)

How can cos A be isolated like this and please show the steps.

thanks

Start
64 = 36 + 49 - 2(6)(7) cos A

0 = -64 +36 + 49 - 2(6)(7) cos A

So: -2(6)(7)Cos(A) = =-64+36+49

Then Cos(A) = (-64+36+49)/((-2)(6)(7))

Simply the addition and multiplication.

Start by replacing cos(A) with x ==> makes it easier to see
64 = 36 + 49 -2(6)(7) x
subtract 64 from both sides and add 2(6)(7)x to both sides
==> 2(6)(7)x = 36 + 49 - 64
==> x = (36 + 49 - 64)/(2(6)(7))
Replace the x with cos(A)
==> cos(A) = (36 + 49 - 64)/(2(6)(7))

64 = 36 + 49 - 2(6)(7) cos A = 85 - 84*cosA

84*cosA = 85 - 64 = 21

cosA= 21 / 84 = 1/4 = 0.25

A = arccos(0.25) = 75.5 degrees >==============< ANSWER

Why all this ? ....... Please LEARN Algebra .......

That's all their is to it, there's no other step