Question

truss analysis
2j = m + r (solve for r)
j = number of joints
m = number of members
r = number of reaction forces

Explanation:

Step1: Isolate R

Subtract M from both sides of the equation.
$2J - M= M + R - M$

Step2: Simplify

On the right - hand side, $M - M = 0$.
$R = 2J - M$

Answer:

$R = 2J - M$