lets say we invest x dollars in each account. so the total investment is 3x. the interest gained from the 6% account is 0.06x, for the 7% account, it is 0.07x and from the 8% account, it is 0.08x. thus we have

0.06x + 0.07x + 0.08x = 52.50

you can solve for x and hence find the total investment