Question

despejar x en
ax=b(a-x)

Answer 1

Tenemos : 

ax=b(a-x)

Despejamos x

ax = ba - bx

ax + bx = ba

a (a + b) = ba

x (a + b)/ (a + b) = (ba) / (a + b)

x = (ba) / (a + b)

Solución : 

x = (ba) / (a + b)

Answer 2

Despejar x

ax  = b(a - x)
ax = ba - bx
ax + bx = ba        Factoizas sacas factor comun x
x(a + b) = ba
x = ba/(a + b)

Respuesta.
x = ba/(a + b)