often_wrong

A. a=4.8m/s/s

B. a=1.2m/s/s

4x^2+15=x^2+18x

• minus 4x^2 from both sides

15=-3x^2+18x

• minus 15 from both sides

-3x^2+18x-15=0

• take a factor of -3 out of brackets

-3(x^2-6x+5)=0

• sum and product gives factors of 6 and -1
• factorise the brackets

-3(x+6)(x-1)=0

• Null factor theorem means:

x = -6 or 1