If z = (15 + 8i) ÷ (3 - 4i), then |z| = 5/17 2 3(2/5) 5

The most reliable way to evaluate this is to eliminate the complex number from the denominator first:

        15 + 8i       3 + 4i        45 + 24i + 60i - 32           13 + 84i
z = * = =
         3 - 4i          3 + 4i               9+16                             25

Just find the magnitude (abs. val.) of 13 + 84i and then divide the result by 25.