Question 969941
(mx^3+3)(2x^2+5x+2)-(8x^5+20x^4)
=2mx^5+5mx^4+2mx^3+6x^2+15x+6-8x^5-20x^4
=(2m-8)x^5+(5m-20)x^4+2mx^3+6x^2+15x+6
=8x^3+6x^2+15x+6
Comparing the coefficients of x^5 we get
2m-8=0
2m=8
m=4
The value of m is 4.