Solve for X, by manipulating equation, making sure to do the same to each side at every step:
2X = (X+10) x 10 - (X-10) x 10
[divide both sides by 10]
0.2X = (X+10) - (X-10)
[simplify right-hand side]
0.2X = 20
[divide both sides by 0.2]
X = 100
Finally check this value of X gives the correct result in the original equation:
LHS = 200
RHS = (110) x 10 - (90) x 10 = 200