If f(x) = x2 − 1, and f(2a) = 35, then what could be the value of a ?

F(2a)=35
35=(2a)^2-1
35=4a^2-1
minus 35 from both sidse
0=4a^2-36
facor out 4
0=4(a^2-9)
factor differece of 2 perfect squares
0=4(a-3)(a+3)
set to zero
a-3=0
a=3

a+3=0
a=-3

a=-3 or 3