we have
4−5=−12 -----> equation 1
y=3 -----> equation 2
Solve the system of equations
substitute the value of equation 2 in equation 1
4x-5(3)=-12
solve for x
4x-15=-12
Adds 15 on both sides
4x-15+15=-12+15 ----> by addition property of equality
simplify
4x=3
Divide by 4 on both sides
4x/4=3/4 -----> by division property of equality
x=3/4