Can you help me to solve equation....

((∂²u)/(∂x²))(x,t)=(1/(c²))((∂²u)/(∂x²))(x,t)

Use the equation above to determine the dimension of the constant c

But why ((∂²u)/(∂x²))=LL⁻²and ((∂²u)/(∂x²))=LT⁻²........

How can it evolution?

Please help me to solve