To solve the equation у+7*(у-3.1) = 8у-21.7, we need to distribute the 7 on the left side of the equation:

у + 7у - 21.7 = 8у - 21.7

Now simplify the equation by combining like terms:

8у - 21.7 = 8у - 21.7

Since there are equal terms on both sides of the equation, the solution is all real numbers.