y = c1e2x + c2e5x .......(1)
Differentiating twice w.r.t. x, we get

The equations (1), (2) and (3) are consistent in c1e2x + c2e5x
∴ determinant of their consistency is zero.
'

This is the required D.E.
Alternative Method:
y = c1e2x + c2e5x
Dividing both sides by e5x ,we get

Differentiating w.r.t. x, we get

Dividing both sides by e-3x, we get

Differentiating w.r.t. x, we get

This is the required D.E.