Maths (finding value)

Question

Could I receive help with questions like below, because I have encountered with many of these and I don't know how to solve them.

 

'Solve n^2-n-90=0 to find the value of n'

 

Thank you in advance.

Answer 1

n^2-n-90=0

you want two numbers that multiply to make -90 but add to make -1, the coefficient of n

-10 and 9 are the numbers

factor

(n-10)(n+9)=0

n-10=0

n=10 (one answer)

n+9=0

n=-9 (second answer)

note: if two numbers multiply to make 0, then one or both must be equal to 0