4945.sagews

var('x,fc')
d=36
n=8
b=12
Pu=90
Mu=700*12
As=4
As1=2
d=36
d1=3
eqns=((d-x)/x*fc*n*As-1/2*b*fc*x-(x-d1)/x*fc*(n-1)*As1+Pu,1/2*fc*b*x*(d-x/3)+(x-d1)/x*fc*(n-1)*As1*(d-d1)-Mu)
solve(eqns, x, fc)

[[x == -157.632743363, fc == -0.100871769507], [x == -27.2008238929, fc == -1.22767648434], [x == 12.8335664336, fc == 3.00359195402], [x == 0, fc == 0]]