Can someone explain to me Linear Equations being equivalent? Why isn't x = -2 and x^2 = 4 equivalent? I don't understand why it is not. Additionally x = 7 and x^2 = 49 is also not equivalent and I cannot figure out why.

Thanks in advance!

Darcy deposited $390 into a savings account. as long as the account balance remains at or above $150, she does not have to pay a service fee for the account. for how many months can she withdraw $40 a month without having to pay service fees? a) 3 b) 5 c) 6 d) 9