sin2 x sin 5x + cos 2x cos 5x = cos 3x
this the education please help me because I have an assignment

simply use the formula: cos(A+B) = cos(A)cos(B) - sin(A)sin(B)

in your question let A = 5x and B = -2x (because 5x - 2x = 3x)

so cos (5x+ (-2x)) = cos(5x)cos(-2x) - sin(5x)sin(-2x)
where, cos(-2x) = cos(2x) and sin(-2x) = - sin(2x)

Simply substitute in the above equation you get:

cos (5x + (-2x)) = cos(5x)cos(2x) + sin(5x)sin(2x)
==> cos (3x) = sin(2x)sin(5x) + cos(2x)cos(5x)

(to prove equality start with the simpler side and expand)