At what point is the tangent to f(x)=x^2 4x-1 horizontal denominator or numerator

Tyrone’s hourly wage is $18 and his net pay is 72% of his earnings. tyrone spends about $1,800 on his monthly expenses. if tyrone works 40 hours per week and has no other sources of income, what is his total monthly cash inflow? (hint: assume that there are 4 pay periods per month.)