The rational root theorem tells you they will be of the form ...
... ±(divisor of 12)/(divisor of 20)
Hence any rational root will be one of ...
... {±1/20, ±1/10, ±3/20, ±1/5, ±1/4, ±3/10, ±2/5, ±1/2, ±3/5, ±3/4, ±4/5, ±1, ±6/5, ±3/2, ±2, ±12/5, ±3, ±4, ±6, ±12}
A graph shows them to be -4/5 and +3/4.
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Dividing (5x+4) and (4x-3) from f(x) gives a quotient of (x²+1). Since there are only two real zeros, we already know the remaining roots are complex. This factor tells us they are 1±i.