(y+a)(y-a)-(5y²-a²)

To simplify the expression \((y + a)(y - a) - (5y^2 - a^2)\), we can first expand the first part of the expression:

1. Use the difference of squares: \[ (y + a)(y - a) = y^2 - a^2 \]

So, the expression becomes: \[ y^2 - a^2 - (5y^2 - a^2) \]

2. Distribute the negative sign: \[ y^2 - a^2 - 5y^2 + a^2 \]

3. Combine like terms:

   • \(y^2 - 5y^2 = -4y^2\)
   • \(-a^2 + a^2 = 0\)

Thus, the simplified expression is: \[ -4y^2 \]

So the final answer is: \[ -4y^2 \]