Evaluate the expression when
x= - 4 and y=7
9y-x?

Okay what you need to do is substitute what it tells you into your equation
so if x= -4 and y=7 then the equation will look like: 9(7)-(-4)

so we can times nine by seven to make it more simple then we have
63-(-4)

when there are two - next to each other, they typically turn into a + so
63+4= 67

I hope this helps