Using Newton's 2nd Law: sumF=m*a and it's rotational analog: sumt = I*a = rF, solve for I by eliminating F to get I_exp = (r^2m(g-a))/a Can someone help me understand how to do this? I am quite lost.

Currents in dc transmission lines can be 100 a or higher. some people are concerned that the electromagnetic fields from such lines near their homes could pose health dangers. for a line that has current 150 a and a height of 8.0 m above the ground, what magnetic field does the line produce at ground level? express your answer in teslas and as a percentage of the earth’s magnetic field, which is 0.50 g. is this value cause for worry?