(3x - a)(x - 2)(x - 7) = 3x³ - 32x² + 87x - 70

It must be true for every value of x, so pick any
value for x and substitute it.  The easiest value
of x we can pick is 0, so let's substitute 0 for x:

(3·0 - a)(0 - 2)(0 - 7) = 3(0)³ - 32(0)² + 87(0) - 70

(-a)(-2)(-7) = -70
        -14a = -70
           a = 
           a = 5

That's the answer.  But you could have picked ANY number
to substitute for x.  For example I could even have picked
a stupid number like 8

 (3x - a)(x - 2)(x - 7) = 3x³ - 32x² + 87x - 70
(3·8 - a)(8 - 2)(8 - 7) = 3(8)³ - 32(8)² + 87(8) - 70
         (24 - a)(6)(1) = 3(512) - 32(64) + 696 - 70
            (24 - a)(6) = 1536 - 2048 + 626
              6(24 - a) = 114
Divide both sides by 6
                 24 - a = 19
                     -a = 19-24
                     -a = -5
                      a = 5

Edwin