There will be $4800$ foxes after $45$ years.
Work Step by Step
There are 3 15 year periods in 45 years, so $x = 3$. According to the order of operations, we simplify inside parentheses, then we simplify powers, then we multiply and divide, and finally, we add and subtract. When we do this, we find: $y=75 \cdot 4^3 = 75 \cdot 64 = 4800$