4*x^2-81=
4*x^2-81=?
Move 81 to other side of = sign and change its sign from -81 to +81 as follows:
4* x^2=81. Now we divide both sides by 4, we have
x^2=81/4 . Now we take the square root of both sides and we get:
x = 9/2 or
x =4 1/2
