help me with this math poblem

1> Expand out the second derivative of y. Each time you take a derivative, you get a pi outside the function. That is where the pi squared is coming from.
2> Add in the (4𝜋^2) 𝑦 term and combine terms.
3> Remember that the sin^2((𝜋𝑥) + cos^2((𝜋𝑥) = 1. This causes all the trig terms to drop out.