a) -(3x-2b)(3x+2b)

b) (x+2)(x-3)(x+7)

Just mutiply out each pair of terms that are in separate brackets.

(a+b)(c+d) = ac+bc+ad+bd

for the three term one, just multiply it out in stages:

(a+b)(c+d)(e+f) = (ac+bc+ad+bd)(e+f) = ace+bce+ade+bde+acf+bcf+adf+bdf

Then you can collect similar terms and simplify.

Hope this helps. Let me know if you are still not following.