I need an equation for this problem I invest 11,000 in an account that pays 10% simple interest. How much adititional money must be invested in an anccount that pays 13% simple interest so that the total interest is equal to the interest on the two investments at a rate of 11%

All I need is help with the equation, from there I can do the work. I just have problems trying to come up with formulas, equations and such.