Let's simplify the given expression step by step:

cos(7π/2 + a) = cos(3π/2 + 2π + a) = cos(3π/2 + a) = -sin(a)

tan(π/2 - a) = cot(a) [using the trigonometric identity tan(x) = 1/cot(x)]

sin(π/2 - a) = cos(a) [using the trigonometric identity sin(x) = cos(π/2 - x)]

cot(3π/2 - a) = tan(a)

Now, substituting these simplifications back into the original expression, we get:

-sin(a) * cot(a) - cos(a) + tan(a)

= -sin(a) * cot(a) - cos(a) + sin(a)/cos(a)

= -sin(a) * cot(a) - cos(a) + sin(a)/cos(a)

= -csc(a) * cot(a) - cos(a) + csc(a)

Therefore, the simplified expression is: -csc(a) * cot(a) - cos(a) + csc(a)