36+2×[3+(a-3)×2]-3=51
36+2×[3+(a-3)×2]=51-3
36+2×[3+(a-3)×2]=48
2×[3+(a-3)×2]=48-36
2×[3+(a-3)×2]=12
3+(a-3)×2=12:2
3+(a-3)=6
a-3=6-3
a-3=3
a=3+3
a=6
