75000=3000/1.2-? could mean 75000=3000/(1.2-?) or 75000=(3000/1.2)-?.
I'm assuming it's the first of these. Replace the question mark by x:
75000=3000/(1.2-x), multiply both sides by 1.2-x:
75000(1.2-x)=3000, expand the parentheses (distribute 75000):
90000-75000x=3000, add 75000x to each side:
90000=3000+75000x, subtract 3000 from each side:
87000=75000x, divide both sides by 75000:
87/75=x, so x=87/75 which reduces to x=29/25=1.16.
Another way:
75000=3000/(1.2-x), divide both sides by 3000:
25=1/(1.2-x), invert both sides:
1/25=1.2-x, convert 1/25 to decimal:
0.04=1.2-x, add x to each side:
x+0.04=1.2, subtract 0.04 from each side:
x=1.2-0.04=1.20-0.04=1.16.
Both methods produce the solution x (or ?)=1.16.