21=3r+-5r

Solution for 21=3r+-5r equation:

Simplifying
21 = 3r + -5r

Combine like terms: 3r + -5r = -2r
21 = -2r

Solving
21 = -2r

Solving for variable 'r'.

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

Add '2r' to each side of the equation.
21 + 2r = -2r + 2r

Combine like terms: -2r + 2r = 0
21 + 2r = 0

Add '-21' to each side of the equation.
21 + -21 + 2r = 0 + -21

Combine like terms: 21 + -21 = 0
0 + 2r = 0 + -21
2r = 0 + -21

Combine like terms: 0 + -21 = -21
2r = -21

Divide each side by '2'.
r = -10.5

Simplifying
r = -10.5