@PaulaPolly,
a) The number of quarterly payments is t*4. The quarterly interesting rate is 7.5%/4. The amount of money after T years is g = $2,200*(1+7.5%/4)^(t*4). The effective yield is the yield after one year, so just compute (1+7.5%/4)^4
b) The function for continuous interest is h = exp(I * t) where I=interest rate or 6.5% and t is the time in years. Once again, for the annual growth factor, put in t=1 year.
c) Graph these two against each other and find when (b) is greater than (a). Remember that (a) has steps to it. You only get interest every quarter. (b) is a smooth line.