To simplify the expression, we will use the angle difference identities for cosine and sine:

Cosine angle difference identity: cos(a - b) = cos(a)cos(b) + sin(a)sin(b)Sine angle difference identity: sin(a - b) = sin(a)cos(b) - cos(a)sin(b)

Now, let's simplify the given expression step by step:

Cos(a - 90) = cos(a)cos(90) + sin(a)sin(90)
= cos(a)(0) + sin(a)(1) [Since cos(90) = 0 and sin(90) = 1]
= 0 + sin(a)
= sin(a)

Sin(a - 180) = sin(a)cos(180) - cos(a)sin(180)
= sin(a)(-1) - cos(a)(0) [Since cos(180) = -1 and sin(180) = 0]
= -sin(a)

Therefore, the simplified expression is:

sin(a) - sin(a) = 0