Answer:
The objective for solving equations for variables is to get the x terms on one side and the non-x terms on the other side.
7) r + sx = t
sx = t - r (Subtract r from both sides)
x = (t - r) / s (Divide both sides by s)
8) m = 2x + n
m - n = 2x (Subtract n from both sides)
(m - n) / 2 = x (Divide both sides by 2)
x = (m - n) / 2 ("Flip" the equation, this is optional but this is how you usually write your answer, it will be in the form x = something.)
9) 4x - 5c = 3c
4x = 8c (Add 5c to both sides; 5c + 3c = 8c)
x = 2c (Divide both sides by 4; 8c / 4 = 2c)