3.9=r^2

Solution for 3.9=r^2 equation:

Simplifying
3.9 = r2

Solving
3.9 = r2

Solving for variable 'r'.

Move all terms containing r to the left, all other terms to the right.

Add '-1r2' to each side of the equation.
3.9 + -1r2 = r2 + -1r2

Combine like terms: r2 + -1r2 = 0
3.9 + -1r2 = 0

Add '-3.9' to each side of the equation.
3.9 + -3.9 + -1r2 = 0 + -3.9

Combine like terms: 3.9 + -3.9 = 0.0
0.0 + -1r2 = 0 + -3.9
-1r2 = 0 + -3.9

Combine like terms: 0 + -3.9 = -3.9
-1r2 = -3.9

Divide each side by '-1'.
r2 = 3.9

Simplifying
r2 = 3.9

Take the square root of each side:
r = {-1.974841766, 1.974841766}